1. Open Steam click View → Servers
2. In the bottom right, click Add A Server
3. Type in the IP and port of your server. For the port, use port+1. (if your game port is 27000, use 27001)
You'll be able to join the server from this list.
1. Open ARK and launch into any singleplayer world
2. Press tab
3. Type: open YourServersIP:YourPort
replacing YourServersIP
and YourPort
with your server's information.
4. If your server has a password, type the password like so: open YourServersIP:YourPort Password