You can add a content editor webpart onto your default.aspx and add
<script language="javascript">
window.location.href = "[your desired URL]";
</script>
But then, your default.aspx will not be accessible at all, because everytime you hit it, you will be redirected. If it’s not the desired functionality then I would recommend adding a cookie to the user’s browser before redirecting. And every time the default.aspx is hit again, first check for a cookie (it will be an indicator if users have already been on the page). If cookie exists, do not redirect, otherwise set the cookie and redirect.