Hello V Rao,
Adding coding like that in the GET_P-method of you attribute will raise and event:
method GET_P_ORDER_REASON.
CASE iv_property.
WHEN if_bsp_wd_model_setter_getter~fp_server_event.
rv_value = 'order_reason_selected'.
ENDCASE.
endmethod.
This event can be handled with an event handler and then you do your logic.
Best regards,
Thomas