Running postgres with a different user/group

I installed latest postgres from sources inside a custom environment of mine.
This environment runs various services, all under a common user/group.
I need to run postgres under the same user/group, witout having to create and use postgres user.
I could do it, and postgres starts.
But now I can't seem to connect to the database, because it says:

FATAL: role "postgres" does not exist

How can I let postgres run under my own user instead of postgres?
I can rebuild from sources, if needed.

Thanx,
Gabriele.

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