There is only a small error in what you have done.
Remove the following line in the After Update event of the Combo Box.
'Private Sub ComboMemberID_AfterUpdate()'
Click the build button (...) of the After Update event, which takes you to the code editor.
The Code is already there with a small error. In the following line remove the space between Square Bracket & ComboMemberID which I have marked in red
Str(Nz(Me![ ComboMemberID], 0))