Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hi Jindřich Šmíd,
Actually, It's not a good practice to use an A record to redirect to Portal.office365.com, potential issue you will encounter one day if the IP is changed or moved.
Given your requirement, the URL redirection service is a better choice, many hosting providers offer URL redirection services, if you manage your domain DNS at your DNS host, search for “redirection” (also called “301 redirection”) at your DNS host’s website, and follow the steps there to set up forwarding.
Thanks,
Frank