@dev43 I think you might be referring to the client_id that is required as part of the POST body when using the POST Invitations Endpoint. If that is the case, when inviting a user to an organization, you need to specify the client_id of an interactive (SPA, Regular Web App) application. This would be the application that you are inviting a user to use. Otherwise, if you had multiple applications served from your Auth0 tenant, there would be no way to know which one you want the user to see when accepting the Organization invitation. See: Invite Organization Members