How to copy an item on a button action?

Hello,

I am new to APEX 4.02, and am failing on what should be very simple.  I want to add a button that will copy the value from one item to another, such as :P1_item_1 to :P1_item_2.

When I click the button, the value copies, but then the screen flashes, and the target value is cleared.  I assume the page is being submitted, and the original null value from the database is returned into the item again, but I am unsure.

This may be due to the Button Request fields, and I do not know what to do for these.  Or maybe the Event Scope, which is set to once.  I am trying to use a Dynamic Action with a PLSQL Function body, with "RETURN :P24_SUPERVISOR_ID ;" in it.