Hi,
Define the server round-trip in the get_p_BILLTO_ADDRESS_SHORT method , So that when you select any value from the drop down screen get refreshed.
add below code :
WHEN IF_BSP_WD_MODEL_SETTER_GETTER=>FP_SERVER_EVENT.
RV_VALUE = 'BILTO_PARTYID'.
Now in the SET_BILLTO_NAME method add the code to get the bill to party id from the address and pass it to the standard code i.e.
SET_BILLTO_ID( ATTRIBUTE_PATH = 'BILLTO_ID' ITERATOR = ITERATOR VALUE = billto party ID).
Thanks
Kumar