Just open it with notepad (on windows), any text editor. Add to the very last line:

RF<space><space><space><space><space><space>3<space><space><space><space><space><space><space><space>

Replace the 3 with how many employee W2 records are submitted. For me, it was 3.

Replace <space> with actual spaces.

I uploaded a test and it seemed to work.