SSH over RDS?

This is the opposite of the usual "tunnel xxxx service through SSH"

I'm looking for a way to tunnel THROUGH an RDS server, in order to make outbound SSH connections.

I've done some research and found some advice on creating a new collection in RDS, and a separate FQDN and port.

But this doesn't answer how to authenticate against the RDP service through SSH (I don't want an open proxy tunnel)