Mailchimp's Marketing API lives at https://<dc>.api.mailchimp.com/3.0, where <dc> is your data center — the suffix after the dash in your API key (e.g. us21).
Create a key under Account → Extras → API keys. Send it as Authorization: Bearer <key>. In the examples it's written {{MAILCHIMP_API_KEY}}, and {{DC}} is the data center.
Full reference: https://mailchimp.com/developer/marketing/api/
curl https://{{DC}}.api.mailchimp.com/3.0/ping \
-H "Authorization: Bearer {{MAILCHIMP_API_KEY}}"Mailchimp's API still calls audiences lists.
curl "https://{{DC}}.api.mailchimp.com/3.0/lists?count=10" \
-H "Authorization: Bearer {{MAILCHIMP_API_KEY}}"curl -X POST https://{{DC}}.api.mailchimp.com/3.0/lists/{{LIST_ID}}/members \
-H "Authorization: Bearer {{MAILCHIMP_API_KEY}}" \
-H "Content-Type: application/json" \
-d '{"email_address": "[email protected]", "status": "subscribed"}'