At Selection Screen On Help Request

At Selection Screen On Help Request event triggers when we press F1 button for a particular field.

INITIALIZATION.
  SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME.
  PARAMETERS p_bsart TYPE ekko-bsart.
  SELECT-OPTIONS: s_ebeln FOR ekpo-ebeln,
                  s_ebelp FOR ekpo-ebelp.
  SELECTION-SCREEN END OF BLOCK b1.

AT SELECTION-SCREEN ON HELP-REQUEST FOR p_bsart.
  MESSAGE 'No document on Purchase Type' TYPE 'I'.