Pop-up message only upon report execution

Dear forumers,

I will need to generate a pop-up message for a report with the following requirements:-

- If a combination of certain selection-screen elements are NOT filled up at the selection screen, generate pop-up message with two buttons, CONTINUE and CANCEL.

- This pop-up message should only be generated upon executing the report / via F8 key.

- The pop-up message must be displayed with the selection-screen still in the background.

- If the user clicks on CANCEL, the report will not be executed and the selection screen should be shown with all of its values intact as before.

- If the user clicks on CONTINUE, the report will be executed, eventually displaying an ALV output.

To achieve this, I used the function module, POPUP_TO_CONFIRM in the event, AT SELECTION-SCREEN but with the condition, SSCRFIELDS-UCOMM = 'ONLI'.

AT SELECTION-SCREEN.

  IF SSCRFIELDS-UCOMM = 'ONLI'.
    IF   R_MATNR IS INITIAL  AND
       ( R_AUFNR  IS INITIAL AND R_AUART  IS INITIAL AND
         P_SELID  IS INITIAL AND R_OBJNR  IS INITIAL AND
         R_OUTDEL IS INITIAL AND R_QMNUM  IS INITIAL AND
         P_NTWK_L IS INITIAL AND P_ACTV_L IS INITIAL AND
         P_NTWK_H IS INITIAL AND P_ACTV_H IS INITIAL AND
         P_REVNR  IS INITIAL AND R_POSTP  IS INITIAL AND
         R_COSTC  IS INITIAL ).
      CLEAR LV_ANSWER.

*     display pop-up message
      CALL FUNCTION 'POPUP_TO_CONFIRM'
        EXPORTING
          TITLEBAR              = TEXT-095
          TEXT_QUESTION         = TEXT-096     " Performance-related text message
          TEXT_BUTTON_1         = TEXT-097     " CONTINUE
          TEXT_BUTTON_2         = TEXT-098     " CANCEL
          DEFAULT_BUTTON        = '2'
          DISPLAY_CANCEL_BUTTON = ' '
          IV_QUICKINFO_BUTTON_1 = TEXT-099
          IV_QUICKINFO_BUTTON_2 = TEXT-098
        IMPORTING
          ANSWER                = LV_ANSWER
        EXCEPTIONS
          TEXT_NOT_FOUND        = 1
          OTHERS                = 2.

*     check return code
      IF SY-SUBRC = 0.
        IF LV_ANSWER <> '1'.
          LEAVE SCREEN.
        ENDIF.
      ENDIF.
    ENDIF.
  ENDIF.

Problem:

- A user runs the report for the first time, displaying the selection screen.

- He hits the 'Enter' key at any of the selection screen elements - nothing happens, no pop-up message appears.

- Once he hits the 'F8' key / executes the report and if a combination of certain selection-screen elements are NOT filled up at the selection screen, the pop-up message appears (with the selection screen in the background).

- If he chooses the CANCEL option, the report will not be executed and the selection screen is shown again with its values intact as before.

- But from this point onwards, whenever he hits the 'Enter' key at any of the selection screen elements again, the same pop-up message appears - it should only appear upon the 'F8' key / report execution only. How can I better control this?