FYI this is what I did
In 'Lodgers'
Private Sub cmdAddNewPayment_Click()
DoCmd.OpenForm "Payments", acNormal, , , acFormAdd, acDialog, "Edit"
End Sub
In 'Payments'
Private Sub Form_Open(Cancel As Integer)
If Me.OpenArgs() = "Edit" Then
cmdEditPayment_Click
End If
End Sub