Restrict VPN to certain ports

[Reactor5]

I'm trying to VPN in somewhere and it doesn't like any outbound connections. I'm doing this for RDP, so can I somehow restrict the VPN connection to only be using the RDP port?

10.04

[Reactor5]

So I guess I'd use the routes panel for this, but I can't find any documentation about it. Can someone give me a pointer to the docs? I just can't seem to find anything more recent that 7.10 :\