It's a two step process
1. Add UF to Project
2. Add UF to UserForms collection to .Show
FWIW, I've found it more reliable to use the item number in the UserForms collection
Option Explicit
Sub test()
Dim ufPld As VBIDE.vbcomponent
'add to project
Set ufPld = ThisWorkbook.VBProject.VBComponents.Add(vbext_ct_MSForm)
'add to userforms collection
Debug.Print VBA.UserForms.Count
VBA.UserForms.Add ufPld.Name
Debug.Print VBA.UserForms.Count
'show using item number
VBA.UserForms(VBA.UserForms.Count - 1).Show ' Zero based so need to subtract 1
End Sub