Using Linq's code...
Try this in the control's Got Focus event and tab into the field. I am guessing you have a mask for the field/control, so I would like to test the behavior. Try it with a value and without a value in the field.
You can create a Got Focus event by highlighting the control and going to the Properties window. Click the Event tab. Click the Elipses(...) next to the Got Focus field. Click Code builder. Your VBA IDE will open. Paste the code below within the Event Handler you just created.
Code:
If IsNull(Me.zip_code.Value) Then
Me.Mzip_code.SelStart = 0
End If
If Not IsNull(Me.Mzip_codeValue) Then
If Len(Me.zip_code.Value) < 20 Then
Me.zip_code.SelStart = 1
Else
Me.zip_code.SelStart = 0
End If
End If