url has been blocked by CORS policy

I know I am not the only one with this problem and I have already read many articles on the subject in the last two days, especially in this community. Unfortunately, I have not yet found a working answer for my case. Therefore I try to post my code here and hope for help.

I am trying to add a profile to a list using fetch(). The site is ssl secured. 

Here is my code:

const api = "123456789123456789";
const list_id = "LIstId";
const url = `https://a.klaviyo.com/api/v2/list/${list_id}/members?api_key=${api}`;


var myHeaders = new Headers();

myHeaders.append("Accept", "application/json");
myHeaders.append("Content-Type", "application/json");
myHeaders.append("Access-Control-Allow-Origin", "*"); 



var raw = JSON.stringify({
            "profiles": [
                {
                "first_name": "Henry",
                "last_name": "Dust",
                "email": "henry@dust.com"
                }
            ]
            });



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




fetch(url, requestOptions)
          .then(response => response.text())
          .then(result => console.log(result))
          .catch(error => console.log('error', error))

But all i get is:

Access to fetch at 'https://a.klaviyo.com/api/v2/list/ListId/members?api_key=123456789123456789' from origin 'https://liveurl.com' has been blocked by CORS policy

Any help is appreciated, I'm stuck here for 2 days now.

Thank you