InfoPath Dev

I have added a TexBox control and set its Properties value to 'UserName' using Function button. 

When I 'Preview' the form; it does display username in it but when I launch application; the TexBox is blank.

How do I get it to display the UserName in the TexBox control?

I checked that it works for me in 2010 upon publishing to lists and libraries

Though in Infopath filler/client form it shows the short username from Windows of client machine (in my case Administrator) and in browser form the sharepoint username (in my case i:0#.f|membership|[email protected])

 Have you published the template after having set textbox Properties value to 'UserName' using Function button and creating a new template with a new template? Default value will not show up create a form it with an earlier (published) template or edit the earlier saved/submitted form with already saved (for example, blank) value of text box.  

You would probably want to set the textbox value  with a rule on Form Load (under Data tab in ribbon) instead of default value. 

Is it 2007 or 2010?

I am on 2007 and don't see the options you mentioned above.