Assign contacts to account

Attaches one or more contacts to an account (max 50 per call). Every ref in the body is resolved within your firm, so a ref you do not own fails the whole request rather than being partially applied. After assigning, an account with more than one contact becomes JOINT. Re-assigning an already-attached contact returns 409.

Your organisation is derived from your API key. There is no field for naming an organisation or a CRM, and any unrecognised field in the body returns 400 rather than being ignored — so a misunderstanding surfaces immediately instead of silently doing nothing.