Assuming you mean a VBA Userform and not a Word Form, you need to code
UserFormName.Show in either an AutoOpen macro or a Document_Open Event
macro.
AutoOpen macros can go in any standard code module (just call the procedure
AutoOpen). Document_Open Event macros must go in the ThisDocument class
Module (select Documnet from the left hand dropdown in the main VBE code
window, and then Open from the right hand dropdown). Both may go either in
your document or its Template, depending on requirements.