Hi,
I would say instead of adding this label you just set the modalpopup extender label value and show
modelpopup extender using the show() method, make sure to set onClientClick=return false or set that button as okcontrolid for modalpopup extendar
Or else you can register startup script also, as you are using ajax in your page you need to user scriptmanager.registerstartupscript, pls refer
ScriptManager.RegisterStartupScript