Hi,
I don't think you can turn off that authentication page because you have to enter a groupID so the system understand that you should use SAML.
You can type https://Server-URL/MyDevice?ac=groupID and you should get SAML authentication directly.
And that you can do (which I have done with some customers), as a workaround, is to configure a reverse proxy or something like that which automatically add ?ac=groupID to the request URI.