Add button on selection screen in toolbar

Check program DEMO_SEL_SCREEN_FUNCTION_KEY.

TABLES sscrfields.

PARAMETERS: p_carrid TYPE s_carr_id,
            p_cityfr TYPE s_from_cit.

SELECTION-SCREEN: FUNCTION KEY 1,
                  FUNCTION KEY 2.

INITIALIZATION.
  sscrfields-functxt_01 = 'LH'.
  sscrfields-functxt_02 = 'UA'.

AT SELECTION-SCREEN.
  CASE sscrfields-ucomm.
      WHEN'FC01'.
      p_carrid = 'LH'.
      p_cityfr = 'Frankfurt'.
    WHEN 'FC02'.
      p_carrid = 'UA'.
      p_cityfr = 'Chicago'.
  ENDCASE.

START-OF-SELECTION.
  WRITE / 'START-OF-SELECTION'.