how to set focus on textbox after post back in asp.net

i have a textbox in update panel. when a user type something i fetch related data from database and fill that in another textbox. My problem is that after autopostback focus on any of the textboxs is lost. How can i manage this using javascript or code because i used both like in code i used ``` System.Web.UI.ScriptManager.GetCurrent(this).SetFocus(this.txtReference); ``` and javascript i find one more that is ``` <script type="text/javascript"> var postbackElement; Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginRequest); Sys.WebForms.PageRequestManager.getInstance().add_pageLoaded(pageLoaded); function beginRequest(sender, args) { postbackElement = args.get_postBackElement(); } function pageLoaded(sender, args) { var updatedPanels = args.get_panelsUpdated(); if (typeof (postbackElement) === "undefined") { alert('if Loop'); return; } else if (postbackElement.id.toLowerCase().indexOf('button1') > -1) { alert('else'); for (i = 0; i < updatedPanels.length; i++) { document.getElementById('<%= txtAcctNo.ClientID %>').focus(); } } } </script> ``` but not working because 'button1 undefined'. What i place there because all event performed on OnTextChanged="" in aspx page. So please help me through code or javascript how can i do this .