I have a legacy application which has a Users table (with user account credentials). I would like to use the DF DataMesh feature to establish a virtual foreign key between this table and the DF User Profile. I believe I must first create a locked-down Service connection to the DF backend, but I cannot get the connection string right. What database name and credential set (root?) must be used? Is this approach even possible? Alternatively, can the User/Profile service exposed via API Docs be customized to include a relation or virtual foreign key to an external service? Thanks for any advice you might have.