Autoselect text in field

Seems like this would be pretty straightforward, but cruising through the API I can't seem to find a clear answer.
 
I want a user to be able to click on a text field, which then autoselects everything in the field. End result is when he starts typing, it automatically replaces the existing text.
 
I tried using "settextselection" with a mouseup action, but I'm pretty new at all this...no dice.
 
Any advice? Just trying to add a bit more user-friendly functionality.
  
Thanks as always!!