To do it all via the userform would entail:
1. replicating the functionality of the Insert Hyperlink dialog in the userform;
2. using either:
(a) separate textboxes for the text input before & after the hyperlink; or
(b) a character sequence in the input text to indicate where the hyperlink should go,
as well as the one for the hyperlink itself
If you make the userform modeless (by setting the form's 'ShowModal' property to false), however, the user can click in the document and add the hyperlink (using Word's own tools) to whatever gets added via the userform. However, that also means they can make other edits too, unless you protect everything else.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]