Modules & VBA :: Pass Value To Form That Opened A Popupform?

I have two subforms in a tab formation sitting on a main form. They can both open up a single popup form (via a button) and that popup form is opened via openargs with the autoID field.

What I need to do is pass back a value on the popup forms [On Unload] event to the subform which opened it.

As the popup form is Modal, the subform which opened it can't be changed.

Am I right in thinking I can pass back the value to the form which had previous focus?

Previously, to pass a value from a popup form to a single form I've been using

Code:

If CurrentProject.AllForms("MyForm").IsLoaded Then
do this
else do this
end if

But in this instance, how do I code the On Unload event in which to tell Access which form opened up the popup form, and pass a value back to it?

I'm guessing
Screen.PreviousControl.SetFocus
has something to do with it?