// To use put this into code behind 
// TextBox1.Attributes.Add("onkeypress", "return clickButton(event,'" + Button1.ClientID + "')");

function clickButton(e, buttonid) { 
      var evt = e ? e : window.event;
      var bt = document.getElementById(buttonid);
      if (bt){ 
          if (evt.keyCode == 13){ 
                bt.click();
                return false;
          } 
      }
}