open new window when click or tab in a text item field, possible?

hi  in 11.5.10.2

Create a custom form.  It has 2 window WinA and WinB.  WinA has 1 text field called Full_name.  WinB has 2 text fields called First_name, last_name.   The requirement is when user click in or tab in Full_name in WinA, WinB will popup/display.  User enter First_name and/or last_name in WinB, click OK, save, close WinB.  Then the First_name || last_name will be populated in Full_name in WinA

Should be easy requirement, but I just cannot find the right trigger or text field property setup.  Any help?