[DotNetDevelopment] Auto-generating a primary key

[BigJ]

I see a formula section, but not sure how to go about it.  I basically
have an primary key called "EntryId" and I want to consist of  UserId
+unique counter, so if UserId="BigJ", and it's my first entry, then
EntryId="BigJ1" and the next entry would be BigJ2 etc....any insight
as to how to accomplish this? I know there is a formula field and I am
looking into it, but any insight or simple examples are apreciated, as
I left my SQL book at home lol...Thanks...