On the Machine 1 (that has Microsoft Active Directory installed), create another user account so that the username is the same as that of one user existing in eXo Platform, for example "root".
Log in the Client machine with the user created in the previous step (root in this case).
In the Client machine, open the http://server.example.com:8080/portal on the browser. Note that you may need to reconfigure the browser as said earlier. As the result, you will be auto-logged into eXo Platform as the "root" user without login again in the Login form of eXo Platform.
Log into Kerberos with the kinit root command, then go to http://server.example.com:8080/portal. You will be auto-logged into eXo Platform as the "root" without login again in the Login form of eXo Platform.
Test the FORM fallback by destroying the ticket with the kdestroy command. Then, log out eXo Platform and log in again. You now can fill user credentials in the eXo Platform login form.
In case you want to enforce authentication only by SPNEGO and disable the authentication through the eXo Platform login form, see the next for how-to.