Disable Button to prevent double click in ASP.Net

I've tried using the solution at https://www.aspsnippets.com/Articles/Disable-ASPNet-button-after-click-to-prevent-double-clicking.aspx but it isn't working in my case.  I believe the problem is that my button is in a panel that is by default not visible.  Here's the Javascript code I'm using: 

window.onbeforeunload = DisableSubmitButton();

function DisableSubmitButton() {
   var btn = document.getElementById('<%=btnSubmit.ClientID%>');

   //make sure submit button was found (it may be in a panel that has Visible set to false). If found, disable it
   if (btn) {
       btn.disabled = true;
   }

}

It isn't finding the button and disabling it when I click it.  Any ideas?