Actually, you CAN! If you're OK with it starting hidden.
Save the workbook as a macro enabled add-in (.xlam)
and use the workbook open event to load up your user form
If you intend to eventually show the workbook, then this may not be workable.
Private Sub Workbook_Open()
Load UFTest
UFTest.Show
End Sub
And your macro code may need to be modified to make sure that you're referencing the workbook/worksheet/range that you intend.
This may prove to be more hassle than it's worth, but give it a shot!
OR, you could use your PERSONAL.xlsb workbook to house the user form. This is generally not visible when you start excel, but the code and forms are useable.
I guess that it all depends on what your user form will be doing and what workbooks it might need to interact with, or whether you'll be distributing the workbook for others to use.... (not advisable with the PERSONAL workbook.)