Thanks for the help. I manage to call up the radprompt and trigger the callback function with the code behind
protected void btnReject_Click(object sender, EventArgs e)
RadAjaxManager1.ResponseScripts.Add(@"radprompt('Reject reason', promptCallBackFn, 330, 160, null, 'Reject', '')");
However in the promptCallBackFn, I am trying to assign the value return to a asp:label and the object is null
var lbl = $find("<%=lblRejectReason.ClientID %>");
lbl.text = args;
I believe it is something to do with Ajax where the control is not loaded, how can I overcome this?