Regarding your specific request, see the below example. It assumes that the form has two fields (Field1 is the email address and Field2 is the subject). Then you can add a custom HTML field with the following:

<button onclick="openEmailEditor()" type="button">Compose Email</button>
<script>
  function openEmailEditor() {
    var email = document.getElementById('Field1').value;
    var subject = document.getElementById('Field2').value;
    window.location.href = "mailto:" + email + "?subject=" + subject;
  }
</script>

If you just want the email address, then you can modify the code and remove the parts about the subject.