Mirrors Meta's POST /{accountId}/customaudiences — the same endpoint creates both, distinguished by the body's subtype.

• Custom Audience (customer list): subtype, customer_file_source. Member upload is a separate call (POST /audiences/{audienceId}/users) with the audience id this returns.
• Lookalike: subtype: "LOOKALIKE", lookalike_spec, origin_audience_id.

Write — needs a write-scoped token.