Question about updating a field on a page layout

I use Custom Object 14 and on its page layout, I have the following fields
1) Name
2) Campaign (picklist)
3) Campaign: External Unique ID

Name is a required field so I can't save the record without populating the Name field. 

After I select a campaign, the campaign: external unique ID field will have a value in it.  I want to populate the Name field with the value in the Campaign: External Unique ID field automatically.

I tried using workflow but it did not work.

Do you know how?

Thanks