@shedman the public key is the SSH key from your computer allowing you to connect to the server from your command line.
So on Forge you should select your server and on the side menu, you will see SSH keys go ahead and add your own. To find it on your computer it should be in ~/.ssh/id_rsa.pup if you are on a Mac, so you need to copy the content of that file, and paste it in the TextArea on forge.
Here is how you can setup SSH keys if you haven't done that before: https://www.digitalocean.com/community/tutorials/how-to-set-up-ssh-keys-2
Then in your TablePlus you will select your server the username is typically forge and just select your public SSH key from your computer, and you should be good to go.