You can place arguments in the Event:FireServer(). When you are writing up your OnServerEvent function, be aware that the player that fired the server is listed before your arguments. Event.OnServerEvent(plr, arguments)
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So lets say you want to transmit the letter x from the client to the server for your encode.
4 | game.ReplicatedStorage.RemoteEvent:FireServer(var) |
Now for the server.
2 | game.ReplicatedStorage.RemoteEvent.OnServerEvent( function (player, var) |
Hope this answered what you were looking for.