Did you try to debug and see how the execution went?
You have not handled the operations required during the callback out here.
This is not the way to do it:
string refscript = this.Page.ClientScript.GetCallbackEventReference(this, "", "scriptevent", null, true);
bttn.Attributes["OnClick"] = refscript;
In correct. You need to register the callbackevent refernece with page.
Have a look at this MSDN example: Client-Callback Implementation (C#) Example
MSDN: Another detailed explanation
Further, have a look at this article with sample. It would help you to dig deeper and learn: ClientCallback custom control for web applications