We need a form, where the customer can order information material, either by email or mail/post.
Depending on the selection in the dropdown (email or mail/post) the either the field "email" or "shipping address" should be visible.

I was able to hide the mail fields when loading the form with this code:
MsCrmMkt.MsCrmFormLoader.on('afterFormLoad', function(event) {
document.getElementById('postanschrift').style.display= 'none';
});

However the I cannot trigger a function upon changing the dropdown. The following code does not work:

document.getElementById('696494fc-72e0-ea11-a813-000d3a387ed9').onchange = function() {
document.getElementById('postanschrift').style.display= 'block';
}

(the ID of the dropdown is correct!)