I adjusted it to:

// AIDEN TO KLAVIYO
      var _learnq = _learnq || [];
    // Listen for events on the webpage
    window.addEventListener("message", function (event) {
    // Check if the event is from Aiden and if it is the 
    // customer info submission event
    if (event.data.advisorId && event.data.type === "customer-info") {
    // Access the event data using event.data
    var email = event.data.data.email
    var checkboxChecked = event.data.data.checkboxChecked
    var advicedproducts = event.data.data.advice

    //KLAVIYO

var myHeaders = new Headers();
myHeaders.append("Content-Type", "application/x-www-form-urlencoded");

var urlencoded = new URLSearchParams();
urlencoded.append("g", "LISTID");
urlencoded.append("$fields", "email");
urlencoded.append("email", email);

var requestOptions = {
  method: 'POST',
  headers: myHeaders,
  body: urlencoded,
  redirect: 'follow'
};

fetch("https://manage.kmail-lists.com/ajax/subscriptions/subscribe", requestOptions)
  .then(response => response.text())
  .then(result => console.log(result))
  .catch(error => console.log('error', error));

  // Custom Klaviyo Event
    var item = {
    "AdvicedProducts": advicedproducts
    };
    _learnq.push(['track', 'Adviced Products',{
    "Adviced Products": advicedproducts
    }]);
    console.log("Adviced Products:", advicedproducts);
    console.log('Adviced Products triggered :)');
   }
 });
// END AIDEN TO KLAVIYO

This makes the subscription to a List working, only the Custeom event “ADVICED PRODUCTS” is attributed to the an other person (active cookie).

For those wondering, below was the solution:

// AIDEN LOVES KLAVIYO

    // Listen for events on the webpage
    window.addEventListener("message", function (event) {
      // Check if the event is from Aiden and if it is the customer info submission event
      if (event.data.advisorId && event.data.type === "customer-info") {
        // Access the event data using event.data
        var email = event.data.data.email;
        var checkboxChecked = event.data.data.checkboxChecked;
        var advicedproducts = event.data.data.advice;
        
        var SalesChannel = document.querySelector('input[name=salesChannel]').value;
        var Language = document.querySelector('input[name=klaviyoLanguage]').value;

        const eventOptions = {
          method: 'POST',
          headers: {
            accept: 'application/json',
            revision: '2024-07-15',
            'content-type': 'application/json'
          },
          body: JSON.stringify({
            data: {
              type: 'event',
              attributes: {
                properties: {
                  AdvicedProducts: advicedproducts,
                  SalesChannel: SalesChannel,
                  Language: Language
                },
                metric: { data: { type: 'metric', attributes: { name: 'Adviced Products' } } },
                profile: {
                  data: {
                    type: 'profile',
                    attributes: {
                      email: email
                    }
                  }
                }
              }
            }
          })
        };

        fetch('https://a.klaviyo.com/client/events/?company_id=COMPID', eventOptions)
          .then(response => response.json())
          .then(response => console.log(response))
          .catch(err => console.error(err));

        if (checkboxChecked) {
        // Mapping SalesChannel to corresponding list IDs
        var listIds = {
          'EN': 'LISTIDHERE_EN',
          'NL': 'LISTIDHERE_NL',
          'DE': 'LISTIDHERE_DE'
        };
        
        var listId = listIds[SalesChannel] || listIds['EN']; // Default to 'EN' if no match
        // Define the subscriptionOptions with dynamic email and other data
        const subscriptionOptions = {
          method: 'POST',
          headers: {
            revision: '2024-07-15',
            'content-type': 'application/json'
          },
          body: JSON.stringify({
            data: {
              type: 'subscription',
              attributes: {
                custom_source: 'Aiden signup form',
                profile: {
                  data: {
                    type: 'profile',
                    attributes: {
                      email: email
                    }
                  }
                }
              },
              relationships: { list: { data: { type: 'list', id: listId } } }
            }
          })
        };

        fetch('https://a.klaviyo.com/client/subscriptions/?company_id=COMPID', subscriptionOptions)
          .then(response => response.json())
          .then(response => console.log(response))
          .catch(err => console.error(err));
        }
      }
    });
  

// AIDEN LOVES KLAVIYO

    // Listen for events on the webpage
    window.addEventListener("message", function (event) {
      // Check if the event is from Aiden and if it is the customer info submission event
      if (event.data.advisorId && event.data.type === "customer-info") {
        // Access the event data using event.data
        var email = event.data.data.email;
        var checkboxChecked = event.data.data.checkboxChecked;
        var advicedproducts = event.data.data.advice;
        
        var SalesChannel = document.querySelector('input[name=salesChannel]').value;
        var Language = document.querySelector('input[name=klaviyoLanguage]').value;

        const eventOptions = {
          method: 'POST',
          headers: {
            accept: 'application/json',
            revision: '2024-07-15',
            'content-type': 'application/json'
          },
          body: JSON.stringify({
            data: {
              type: 'event',
              attributes: {
                properties: {
                  AdvicedProducts: advicedproducts,
                  SalesChannel: SalesChannel,
                  Language: Language
                },
                metric: { data: { type: 'metric', attributes: { name: 'Adviced Products' } } },
                profile: {
                  data: {
                    type: 'profile',
                    attributes: {
                      email: email
                    }
                  }
                }
              }
            }
          })
        };

        fetch('https://a.klaviyo.com/client/events/?company_id=COMPID', eventOptions)
          .then(response => response.json())
          .then(response => console.log(response))
          .catch(err => console.error(err));



        if (checkboxChecked) {
        // Mapping SalesChannel to corresponding list IDs
        var listIds = {
          'EN': 'LISTIDHERE_EN',
          'NL': 'LISTIDHERE_NL',
          'DE': 'LISTIDHERE_DE'
        };
        
        var listId = listIds[SalesChannel] || listIds['EN']; // Default to 'EN' if no match
        // Define the subscriptionOptions with dynamic email and other data
        const subscriptionOptions = {
          method: 'POST',
          headers: {
            revision: '2024-07-15',
            'content-type': 'application/json'
          },
          body: JSON.stringify({
            data: {
              type: 'subscription',
              attributes: {
                custom_source: 'Aiden signup form',
                profile: {
                  data: {
                    type: 'profile',
                    attributes: {
                      email: email
                    }
                  }
                }
              },
              relationships: { list: { data: { type: 'list', id: listId } } }
            }
          })
        };

        fetch('https://a.klaviyo.com/client/subscriptions/?company_id=COMPID', subscriptionOptions)
          .then(response => response.json())
          .then(response => console.log(response))
          .catch(err => console.error(err));
        }
      }
    });