Installation
This plugin works on the backend. No need for a proxy plugin. Simply place the jar file in the plugins folder of any server you want to use portals in, and restart your server. (Note, this plugin does not need to be on the server the player is teleporting to! If you have one central lobby server that has all of the portals, that is the only server that needs the plugin.)

Usage
To register an ordinary into a server switching portal, use the command /register-server *server*, in which you replace *server* with the name of the server you want the portal to transport you to. The *server* parameter must be the exact same that it appears in your velocity.toml (you can find a list of available servers and their exact names with the /server command). You will be promped to walk into the portal you want to register. After you do so, the entire portal (size and shape doesn't matter) will be registered to redirect the player to that server. Multiple portals can be registered to one server!

NOTE:
I highly highly recommend going into the config.yml and changing lobby-mode to true on any server that you configure portals on. This will prevent players from being locked out of a server because they spawn in a portal. It is false by default because this option sends everyone who joins the server to the server spawn when they join, no matter where the are in the world.