how to save a oracle form field values to a database table

Question

BEGIN
  INSERT INTO tbmt_brg_agency_letters
    (ID,
     MTBRG_CREATE_DAT,
     MTBRG_ORNNN_ID,
     MTBRG_RATING_AGENCY_TXT,
     MTBRG_EXTRNL_LTR_RATING_TXT)
  VALUES
    ( MTBLR_SEQ.NEXTVAL,
     :VWMTBRG.CREATE_DAT,
     :VWMTBRG.ORNNN_ID,
     :VWMTBRG.RATING_AGENCY_TXT,
     :VWMTBRG.EXTERNAL_LETTER_RATING_TXT);
  COMMIT;
END;

when i tried to save form values to a table it not saving, I'm using WHEN-BUTTON-PRESSED trigger. I'm trying to insert one table values to other table which are selected using a dropdowm in a form.