Scope: write:address-book

The required fields are the ones Loop's own form requires: firstName, lastName,
companyName, category, consentGiven and at least one email address. category must
be on the list from GET /v2/reference/address-book-categories. The postcode is stored in
capitals.

clientId is only needed when the API key reaches more than one agency (client and
network keys); a team key adds to its own agency. teamId may be sent instead, to add to
that office's agency.

user_id is optional: if supplied it must be a Loop agent in the agency, and is recorded
as the creator and as giving consent; if omitted the agency's first agent is used.