If you had to use a Browser Form rather than opening in the Client you can still work around this.
What we do in my work is add two fields to the Form. One is a check box and the other a normal text box. The text box is set to Read Only and the check box has a rule applied that when ticked sets the text box to the User Function ie. userName()
You may not want to do it this way or you may be happy to have it open in the Client but thought I would post this anyway for anyone else.
Cheers.