PUSHBUTTON and AT SELECTION-SCREEN

Hi, I don't understand why I'm not being able to catch the user command in the example below. I tried in the CASE statement sscrfields-ucomm, sscrfields, sy-comm, but none of them work. Is the problem related with displaying a message ? Or am I indeed misunderstanding the use of the USER-COMMAND assigned to the PUSHBUTTON I created ? All I want is to be able to perform any action in the AT SELECTION-SCREEN event, when the button is pushed.

SELECTION-SCREEN BEGIN OF LINE.
  SELECTION-SCREEN PUSHBUTTON 4(10) but1 USER-COMMAND c1.
SELECTION-SCREEN END OF LINE.

INITIALIZATION.
  but1 = 'BUTTON 1'.
 
AT SELECTION-SCREEN.
  CASE sy-ucomm.
*  CASE sscrfields-ucomm.
*  CASE sscrfields.
    WHEN 'c1'.
      MESSAGE 'You clicked me' TYPE 'I'.
  ENDCASE.

Thanks !

Avraham