InfoPath Dev

I need to pre-populate a drop down list in a InfoPath form with values that are filtered based on current users username.
These values can be retrieved from a web service that accepts a username parameter.
Unfortunately when seeting this parameter's value from Data Connections wizard I can only type in the value and can't use userName() InfoPath function.
How can I set this value to current user?

 You can turn off the "Query this data connection when the form opens" option, then create a rule to run when the form opens that

• Sets the necessary parameter of the web service to the value of the userName() function
• Queries the web service
  

Great, thanks.
And can I create a rule that would run immediately?
Without the need for the user to do anything?