If you have a base table item, Oracle (or Forms) generates SQL at runtime to insert, update, delete and query rows in the base table, based upon user's actions. You don't need to write code for the insert.

With a PRE-INSERT trigger you can assign IDs based on sequences. Set INSERT ALLOWED and NAVIGABLE on FALSE for the item, to prevent entering IDs manually.

SELECT your_sequence.nextval
INTO :block.item
FROM sys.dual;