Hi @meisterSybren!
 

This might be happening because Klaviyo has security measures in place to prevent malicious attacks on their platform by checking the source of the email being added to the form. When you add the email through a URL, Klaviyo might flag it as potentially harmful and remove it.

A solution to this issue would be to use Klaviyo's APIs to pre-populate the form with the email from the URL. Here's a basic example of how you could do this with JavaScript:

<script type="text/javascript">

  // get the email from the URL query string

  var email = getQueryStringValue('email');

  

  // if an email is present, pre-populate the email field

  if (email) {

    document.getElementById("klaviyo_field_0").value = email;

  }

  function getQueryStringValue(key) {

    return decodeURIComponent(window.location.search.replace(new RegExp("^(?:.*e&\\?]" + encodeURIComponent(key).replace(/e\.\+\*]/g, "\\$&") + "(?:\\=("^&]*))?)?.*$", "i"), "$1"));

  }

</script>
You would need to replace klaviyo_field_0 with the ID of the email field in your Klaviyo form.

 

But remember to stick to best practices when sending emails. Make it easy for your subscribers to unsubscribe or change their preferences. Also, be careful not to fall into the spam trap, which can lead to account blocking. 
 

I hope this helped, if you have any other questions, just let me know)