By using a remote event. Remote events are used to safely transfer data from client to server/server to client. Do this :
Client script
local RemoteEvent = game.ReplicatedStorage:WaitForChild("RemoteEvent")
local input = script.Parent.TEXT2
script.Parent.BOX2.MouseButton1Click:Connect(function()
RemoteEvent:FireServer(input.Text)
end)
Server script
local RemoteEvent = game.ReplicatedStorage:WaitForChild("RemoteEvent")
RemoteEvent.OnServerEvent:Connect(function(player, text)
game.Workspace.BOX2.SurfaceGui.TextLabel.Text = input.Text
end)
Now, you can name your remote events to know which one is associated to something specifc. For exemple, you could name this one "BoxText".
For more information : Remote Functions and Events