Select the workspace where your server will belong in Moss and click New server as in the following screenshot.
Choose Existing server at the provider field and type the IP address (or a stable domain name that resolves to the IP address) of your host.
Choose the timezone of your server. Moss will use this value to set such timezone on it. By default Moss chooses the timezone that corresponds to your browser's settings, but you may change it if needed.
Optionally, choose an SSH Public Key to upload onto your server under user
moss. This allows you to log into your server as a privileged user (can run
sudo) later on. But you can do this at any other time.
Provide a name for your server to identify it in Moss. Such name will also become the hostname of the server.
Click Create and finalize the last step: Authorize Moss to connect to your server. For this you must log into it as
root or any other user that can run commands via
sudo. Then copy the script Moss shows you and run it on your server.
If the script succeeds, it'll output the user you're logged in - provide such username in User (output of script) and click Finalize.
Moss will start an operation to set up your server in the background. At this moment, you can keep adding servers or click Close to come back to the main panel of your workspace.
If you click the operations icon at the top of the screen, you should see an in-progress operation to connect your server. Once it's done, you can click Show log to check the details of such operation.