keypress in jquery modal popup

I opened a jquery modal dialog which has 2 textboxes and a button

<table cellpadding="0" cellspacing="0">
        <tr>
          <td align="center"><input name="" type="text" value="" title="User Name" class="width190 enter_popup" id="txtUserName" onfocus="txtFocus(this)" onblur="txtFBlur(this,'0')"/></td>
          <td align="center"><input name="" type="password" value=""  title="Password" class="width190 enter_popup" id="txtPassword" onfocus="txtFocus(this)" onblur="txtFBlur(this,'1')"/></td>
        </tr>
        <tr>
          <td colspan="2" align="center"><input name="" type="submit" class="blue_btn" value="Sign In"  id="btnLogIn" onclick="javascript:return LogIn()"/></td>
        </tr>

      </table>

now in my script I'm calling LogIn function

$(function () {

    $(".enter_popup").keypress(function (e) {
        if (e.keyCode == 13) {
            if ($(this).attr('id') == "txtUserName" || $(this).attr('id') == "txtPassword") {
                LogIn();
            }

        }
    });
 });

function LogIn() {
    var username = $('#txtUserName').val();
    var password = $('#txtPassword').val();
}

but keypress is not fired..any ideas why and what is possible solution for that?

You need to use event-delegation on dynamically created elements

$(document).on('keypress','.enter_popup',function(){
    /*Your code*/
});