[Semibug] ssh to host on local network

[Jonathan Drews]

Hi Folks:

 I want to allow a friend to SSH into a machine that is on a local
network. It is an OpenBSD machine. My internal IP addres as given my
ifconfig is 192.168.11.21:

$ doas /sbin/ifconfig em0
                  
em0: flags=808843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,AUTOCONF4>
mtu 1500
        lladdr 28:d2:44:b2:fb:bf
        index 1 priority 0 llprio 3
        groups: egress
        media: Ethernet autoselect (100baseTX
full-duplex,rxpause,txpause)
        status: active
        inet 192.168.11.21 netmask 0xffffff00 broadcast 192.168.11.255

my routers external IP address is 63.239.66.139:

$ curl https://lathama.net/ip/
63.239.66.139

If I do:

$ ssh guesthere at 63.239.66.139

It can't find the local machine. guesthere is a guest account on the
OpenBSD computer. How do I add in the local  192.168.11.21 address?

I have Michael Lucas' SSH Mastery book but I couldn't find any
examples of ssh into machines on a local network.

My machine is:
$ uname -a
OpenBSD leo.my.domain 7.0 GENERIC.MP#6 amd64

--
Kind regards,
Jonathan