Creating AWS DMS Source endpoint for Postgres version 15?

Unfortunately as you are aware PostgreSQL 15 is not supported by DMS.
I also don't think they have announced when they will support it.
https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Source.PostgreSQL.html

AWS Database Migration Service 3.5.1 now supports PostgreSQL 15.x: https://docs.aws.amazon.com/dms/latest/userguide/CHAP_ReleaseNotes.html