All endpoints are under /api/v1. Send x-api-key with a key issued from the admin dashboard (or a development key when ALLOW_DEMO_API_KEY is enabled).
Minimal request/response path to verify your setup.
curl -sS -H "x-api-key: YOUR_KEY" \
-H "content-type: application/json" \
-d '{"field_key":"countries","value":"NL"}' \
https://YOUR_HOST/api/v1/normalize