DATA:  dynpfields TYPE STANDARD TABLE OF dynpread WITH HEADER LINE.

AT SELECTION SCREEN ON VALUE-REQUEST FOR p1.
   
     dynpfields-fieldname = 'P2'.
     APPEND dynpfields.

     CALL FUNCTION 'DYNP_VALUES_READ'
       EXPORTING
         dyname                               = sy-cprog
         dynumb                               = sy-dynnr
       TABLES
         dynpfields = dynpfields.

      p2 = dynpfields-fieldvalue.