This should work just fine:
Code:
Private Sub Command1_Click()
    Dim lHwnd As Long
    Dim lDesktop As Long
    Dim lSomeOtherApp As Long
    
    lDesktop = GetDesktopWindow
    
    'Here I'm assuming that you already have a
    'window handle of that form and a window handle
    'of the application you want to assign your form to
    
    Call SetParent(lHwnd, lSomeOtherApp)
    
    
    'Do your stuff here
    
    'Now set your form to be a child to a desktop again
    Call SetParent(lHwnd, lDesktop)
End Sub