--_______________________________Client Side_________________________________
local frame = vgui.Create("DFrame")
frame:SetSize(100,100)
frame:SetVisible(true)
frame:Center()
frame:MakePopup()

local b = vgui.Create("DButton" ,frame)
b.DoClick = function()

    net.Start("MessageName")
    net.SendToServer()

end

--_______________________________Server Side_________________________________
util.AddNetworkString("MessageName")