Hi Gita,
'At selection-screen' even is like a PAI and 'At selection-screen output' is like a PBO if you see the anology in screen programming. The second way that you have done should be fine. If you want the output in the same screen you could try putting the condition in 'At selection-screen output' event.
You can initialize F_SUBRC = 8(or any thing other than 0 ) and not display anything initially and then based on sy-subrc value provide the appropriate output(NG,OK,error).
Best regards,
Saurabh