Is there a way to default a value on a lookup record?

I am trying to get a field to default into a lookup prompt table. When the user clicks the field to look up a value I need one field on the list values of the prompt table to default in based on some job criteria. 

This happens to be on the timesheet. So when the user clicks to look up a combo code I need to the Organization to default in based on the employees job.deptid. 

Any ideas on where to put this code?