Connects a supplier account (SRS, ABC Supply or QXO) to your company. Only one provider of each type is allowed, so a second one of the same type fails with 409. Zuper authenticates with the supplier before saving, so the provider is created as CONNECTED only when the supplier accepts the details; otherwise nothing is saved. For SRS, Zuper signs in with its own SRS partner access (send credentials_keys as {}) and verifies the customer account given in customer_data: customer_code plus either invoice_number and invoice_date, or integration_key. For ABC Supply and QXO, send the account's OAuth refresh_token in credentials_keys; Zuper exchanges it for an access token and keeps it refreshed. The new provider becomes the preferred supplier automatically when the company has no active preferred one. The response returns only the new provider's UID. Requires supplier integration to be enabled for the company; otherwise the request fails with 400. Admin only: the API key must belong to an admin user; other roles receive 405 Access Denied.