Embed the invitation code in the email link URL: To implement the redemption process efficiently, the system must pass the unique invitation code from the email to the portal's redemption page. The standard mechanism in Power Pages is to generate a URL in the email body (e.g., https://portal.com/register?invitation={Code}) that contains the code as a query string parameter. This action allows the portal to read the URL and automatically populate the code field on the redemption page, creating a seamless user experience. "Auto-populate" is the result of embedding the code, not the implementation method itself.

Invitation code sign-up: This solution fulfills the requirement to "validate the user's email" within an invitation-based registration flow. When an invitation code is sent to a specific email address, the act of the user retrieving that code and successfully redeeming it (Invitation code sign-up) serves as proof of ownership for that email address. This negates the need for a separate "Email Confirmation" step often required in open registration scenarios. Other options like "Two-factor authentication" or "Azure Active Directory" are authentication methods, not the specific mechanism used to validate email ownership during an invitation-based registration.

Microsoft Learn: Invite contacts to your Power Pages sites - This documentation details the invitation workflow, stating that the email sent to the user must contain a link to the redemption page, and the code is typically passed to facilitate the process.

Source: "Send an invitation... The email contains a link to the redemption page with the invitation code."

Microsoft Learn: Configure authentication in Power Pages - Describes how the invitation model links a contact to a local or external identity, verifying the user's association with the initial contact record (and email).

Source: "Redeeming an invitation allows a user to... associate the contact with a local or external account." (Implies validation of the contact's identity/email via the code).