If you think of manage relational table as helping in a Parent - Relationship - Child database scenario... the Child Fields have to exist and be used to populate a list or checkbox group on the page in order to update the Relationship table properly. You can insert the Parent and manage the Relationship on one page, but you can't also insert into the child since the child has to be used to populate a list or checkbox group for manage relational database to work.
The child table fields have to already exist.