When radio button selected, populate text field with user-entered text from another field

I have a radio button list with five choices. When the second button (called update, value is 2) is checked, I want the text that the user enters in field one to populate text field two.

I can't figure out the Javascript. Action Builder has a "set value of field to another field" option, but that isn't working when I add that condition to the click trigger of the radio button.