Your server

In the Dashboard of the server management panel, select Console.  Click the Start button on the right side. 

Since nothing else has been modified first, this launches a server with default settings.  Give the console a minute to load.  Meanwhile, launch the Vintage Story game the usual way.  In the main menu, select Multiplayer and click the Add New Server button

Give the server a name; it doesn't matter what.  For the Host / IP Address, go back to the console and look for Address a little below the Start/Stop buttons.  Clicking the Address field will copy "vs.suhosting.net:" along with 5 digits that identify the port to your specific server.  Back in the Vintage Story menu, paste (with Ctrl+V) all of that into the Host field.  You can create a password or leave it blank.  Click the Create button to add this server configuration in your list of Multiplayer worlds.  Click on it to load in as a player.

When you're done playing, you can leave the server running, or use the Stop button in the console tab of the server management panel.