The automatic signature feature always inserts the text at the beginning of
the message body. It has no effect on any other fields or controls that
might have been placed on a custom form's design surface.

What exactly do you mean by "overlays the complete form"? If all the
recipient sees is the signature block, that suggests that either you haven't
designed your form with a read layout to match the compose layout or you
have not published the form to the Organzational Forms library or to each
user's Personal Forms library.