set foucus on textbox in ASP.NET application using javascript or jquery

I want to set up the default focus on a textbox in one of my content pages in

For some reason the following code is not working :
       <script lang="javascript" type="text/javascript">
        window.onload = function () {
    function jftnSubmit(evnt, ID) {
        var ButtonID =;
        var btnSearchID = ButtonID.replace('txtFilter', 'btnSearch');

Inside content area 2 :
<input id="txtUserName" runat="server" type="text"
                        class="form-control" placeholder="Username" maxlength="50" onclick="javascript: jftnSubmit(); return false;" />
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Najam UddinCommented:
set autofocus in html of that textbox

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