Javascript to triger submit button

When user clikcs enter insid a textbox, I want to triger the submit button click, Ihave the following code but code inside if is not executed. Please help.

  <script lang="javascript" type="text/javascript">
        window.onload = function () {
            document.getElementById("txtUserName").focus();
        };
        function jftnSubmit(e) {
             if (e.keycode == 13) {
                alert('d enter f');
                var ButtonID = btnLogin.id;
                document.getElementById(btnSearchID).click();
               
             }
             return false;
           
            }      
   
    </script>

 <input id="txtUserName" autofocus  runat="server" type="text"
                        class="form-control" placeholder="Username" maxlength="50" onclick="jftnSubmit(event); " />