How to make a button the default button on enter on an ASP.NET form

function SetDefalutButton(e, buttonid)
{
   var evt = e ? e : window.event;
   if (evt.keyCode == 13)/*Search if user hit enter key*/
   {
      var bt = $('[id$='+buttonid+']');
          if (bt.length > 0)
          {
              if (evt.keyCode == 13)
              {
              eval( bt[0].href);//if it is link button
              //bt.click();//if it button
              return false;
              }
          }
   }
}

Here is how we use it:

<asp:TextBox ID="txtBox" runat="server" 
  onkeypress="return SetDefalutButton(event,'btnTest');" MaxLength="50"></asp:TextBox>