First of all, you need to assign each server an ID (stored inside that server). You can make that ID randomly generated since chance of it repeating is absolutely minimal (remember to put it through filter so it doesnt put inappropiate text).
When a player wants to join a server with given id, use MessagingService to send message like “Hey, looking for a server with id abc123”. It should be received by all servers. If the server receives message, and the id is matching, you can then have it reply with the access code via another message, which you can use to teleport the player.
i often use messaging service , can you show some example script ? also , i don’t think it need messaging service because reserve server just need to get the server code and join . the problem is the code only can get it when teleporting , which mean you need to do something to make the code visible in other server text label . and for some reason , the code is shorten . which i literally don’t know how to do .