Referencing a control on an Access form is easy. Referencing a subform and its controls is a little more complex. To Access, a subform isn't a form at all, but a control, so you must reference it as ...

Create a pop-up form in Access If users need to access data that is not on their forms, you could add a subform; or, you can add a command button that, when clicked, will pop open the form that ...