/gmail/v1/users/{userId}/settings/delegates

Adds a delegate with its verification status set directly to accepted, without sending any verification email. The delegate user must be a member of the same Google Workspace organization as the delegator user. Gmail imposes limitations on the number of delegates and delegators each user in a Google Workspace organization can have. These limits depend on your organization, but in general each user can have up to 25 delegates and up to 10 delegators. Note that a delegate user must be referred to by their primary email address, and not an email alias. Also note that when a new delegate is created, there may be up to a one minute delay before the new delegate is available for use. This method is only available to service account clients that have been delegated domain-wide authority.