How to Teleport to Other Servers by a GUI?

There are two ways to get a player. The first would be to use a DataStore, and the other would be to use MessagingService. I believe that the second option is the best, as it is not too reliant on a DataStore. We are going to be trying to get the JobId of the server that a player of the given UserId is in. Once we have the JobId, we can teleport to that player.

local MessagingService = game:GetService("MessagingService")
local TeleportService = game:GetService("TeleportService")
local Players = game:GetService("Players")

function GetServerData(UserId)
    local PlaceId
    local JobId
    PlayerFoundConnection = MessagingService:SubscribeAsync("PlayerFound"):Connect(function(GivenUserId, GivenPlaceId, GivenJobId)
        if GivenUserId == UserId then
            PlaceId = GivenPlaceId
            JobId = GivenJobId
            PlayerFoundConnection:Disconnect()
        end
    end)
    MessagingService:PublishAsync("GetPlayer", UserId)
    repeat wait() until not PlayerFoundConnection.Connected
    return PlaceId, JobId
end

function TeleportPlayer(Player, PlaceId, JobId)
    TeleportService:TeleportToPlaceInstance(PlaceId, JobId, Player)
end

MessagingService:SubscribeAsync("GetPlayer"):Connect(function(UserId)
    for _, Player in pairs(Players:GetPlayers()) do
        if Player.UserId == UserId then
            MessagingService:PublishAsync("PlayerFound", UserId, game.PlaceId, game.JobId)
        end
    end
end)

As you can see, we have two functions: GetServerData, and TeleportPlayer. These functions can be referenced later on in the script.

Edit: People are saying you need help regarding Discord as well, so let me break that down for you as well. :) Discord has this thing called Webhooks, which are just a way of posting messages on Discord remotely (which in this case, is ROBLOX). A webhook can be added from a server or channel's configuration. I'll be showing you how I do Discord webhooks (with my module, as it makes things simpler).

local Discord = require(3427317493)
local ModCallWebhook = Discord:GetWebhook("YOUR_WEBHOOK_URL")

function PostModCall(Player, Message)
    ModCallWebhook:Post({
        Content = string.format("**Moderator call from %s (%s):\n%s**", Player.Name, Player.UserId, Message)
    })
end