Make sure the field you add to your screen is set as an Editable List Item or Character field.
Inside Screen Builder, from the menu select File|Screen Script and then type:
Code: Select all
If Mode = 3
The above assumes you have called your field "USER". If it is called something else, change "USER" to whatever you have called it.
Using the above approach the user name will be assigned when the operator presses the Save button.