My OnSubmit in my html form doesn't work, it just goes to the URL in the action. I have a vb func...

My OnSubmit in my html form doesn't work, it just goes to the URL in the action.    I have a vb function that should go in the OnSubmit, but even doing False, doesn't work.  
Form is:

form name="aspnetForm" method="post"  onsubmit="return False;" id="aspnetForm" runat="server" action="https://URL" target="_blank"