SQL & PL/SQL :: How To Have Procedure Populate Staging Table Primary Key

I've a staging table STG_TABLEA which has a primary key discount_seq_no.

I am creating a pl/sql procedure to populate a primary key (discount_seq_no) with a database sequence. The intent is to keep this populated with next value incrementing by 1.

I am using Oracle 11.2.0.2 version.

I've put together the below code, not sure on next steps...

BEGIN
UPDATE STG_TABLEA
SET A.DISCOUNT_SEQ_NO = "INSERT A SEQUENCE HERE AND KEEP INCREMENTING the seq value by 1
COMMIT;
EXCEPTION
WHEN OTHERS
THEN
RAISE;
END;