POST to the address shown on the API Documentation page, with the X-API-Key header and a jurisdiction (EU-FR, UK, US, NO or CA).
The response contains either a code or open_question with the question and the codes at stake. The API documentation details the fields.
Multi-country endpoints accept at most 4 destinations per request, among EU-FR, UK, US, NO and CA. Single classification accepts one jurisdiction, Canada included.