Problem statement: How do I specify the Organization when in the process of inviting a user to it?
Solution: Invites can either be sent from the Auth0 Dashboard OR the Management API. Both methods require an Application Login URI be set in the relevant Application settings. This URI needs to point to a route in your application that redirects to your tenant’s /authorize endpoint. It would usually take the form of https://myapp.org/login (this can also be configured to be a call to /authorize directly). To learn more, read Configure Default Login Routes.
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Auth0 Dashboard - The organization and invitation parameters are automatically appended to the Application Login URI when an invite is sent to a user via email from within the specific Organization’s settings.
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Management API - Make a POST call to the Create Organization Invitations endpoint including the ORG_ID among other parameters. The url to POST to will look like: https://YOUR_AUTH0_DOMAIN/api/v2/organizations/ORG_ID/invitations
Resources 
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Related FAQ:
https://community.auth0.com/t/authenticating-users-through-a-specific-organization/84679