tab in or click in a text item field and call a new window

I have a request..  in 11.5.10.2

In a custom form.  It has 2 window WinA and WinB. WinA has one field called Full_name.  it is a text item.  When user click in or tab into the full_name field, WinB pop up.  WinB has 2 text items fields - First_name and last_name.  They both have LOV attached .  When user select or type in the first_name and last_name, click OK.  WinB will close and the first_name || last_name will send to Full_name in WinA.

This request should be simple but some how I cannot finger out

1) what trigger should I use for Full_name in WinA ?    I will call APP_CUSTOM.open_window('WinB');  to open WinB