Capture user initials when box is checked

I want to capture a users initials if they check a certain box.  I didn't want to do this through form rules because the checkbox will be marked in grid edit most of the time.
 
I created the field to show the current user's initials and I wrote a Quick Base Action to copy those initials into a read only text field.  The problem I'm having is that it's not putting in the user's initials. Since I'm the  one that created the Action, QB is automatically putting my initials in the text field, regardless of who is checking the box.

Thoughts on how to get around this?