Joint Field Value Formula to Populate a Field prior to Saving Opportunity

Hello.

I have a field that uses a joint field lookup.  Once the opportunity is saved, is looks up the account to see if it is on our target accout list.  If it is on the list, the prirotiy level is then populated into another field on the opporutity page.

What I am looking to do is to be able to have this field populate as soon as the account is selected, not after it is saved.

Thank you.
Glen