APEX

Hello:

I have a table where my primary key has been set NO for nullable. This field is a system generated value.

When I created my data entry form and I click Insert, I'm receiving the following message:
ORA-01400: cannot insesrt null into (tablename.fieldname).

In the Page Processing section, under Processes, I have an INSERT after Submit process created. I have it set for Automatic Row Processing (DML). When I do this, the screen requires that I identify the primary key. Well, my primary key is this system generated value and it is not generated until after the INSERT should occur. Therefore at the time of the entering the data into the form, the value for the primary key doesn't exist yet.

How do I submit my form with all my data into this table which has a primary key that is system generated and set to NO for nullable?

Can someone help me?
Thanks.