To add a new domain in portal.office.com, log in as an admin and navigate to the Admin Center. From the left menu, select Settings, then Domains, and click Add Domain. Enter your new domain name, then follow the on-screen instructions for domain verification. This typically involves updating DNS records with your domain provider to confirm ownership. Once verified, you can configure services like email and Teams to work with your domain. After setup, the domain will appear under Domains in portal.office.com, allowing you to manage users, email addresses, and custom branding within your Office 365 environment.