Hi,
If you want default flow to use SSO without a click, then indeed change the script to set the cookie to /oauth... You can use the 'manual' login (for as administator) by navigating to login.html yourself.
If you want the users to click the button 'login with SSO', you can keep the script to set the cookie to login.html and just place a button which redirects to /oauth...
Good luck!
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<script>
document.cookie = "originURI=/login.html" + (window.location.protocol === "https:" ? ";SameSite=None;Secure" : "");
</script>