Use a popup code for subscribe form or pure js.

[rene-hermenau]

The content of the subscribe button should be generated via js to prevent indexing from search engines

<script>

/*HTML Code hier in die Variable 'form' hinein */

var form = '<form>  First name:<br>  <input type="text" name="firstname"><br>  Last name:<br>
  <input type="text" name="lastname"></form>';

document.write(form);

</script>