Klaviyo's API at https://a.klaviyo.com/api follows JSON:API and requires a revision header naming the API version (a date).
Create a private key under Settings → API keys and send it as Authorization: Klaviyo-API-Key <key>. In the examples it's written {{KLAVIYO_PRIVATE_KEY}}.
Full reference: https://developers.klaviyo.com/en/reference/api_overview
curl https://a.klaviyo.com/api/accounts \
-H "Authorization: Klaviyo-API-Key {{KLAVIYO_PRIVATE_KEY}}" \
-H "revision: 2026-07-15" \
-H "accept: application/vnd.api+json"
curl "https://a.klaviyo.com/api/profiles?page%5Bsize%5D=10" \
-H "Authorization: Klaviyo-API-Key {{KLAVIYO_PRIVATE_KEY}}" \
-H "revision: 2026-07-15" \
-H "accept: application/vnd.api+json"
curl https://a.klaviyo.com/api/lists \
-H "Authorization: Klaviyo-API-Key {{KLAVIYO_PRIVATE_KEY}}" \
-H "revision: 2026-07-15" \
-H "accept: application/vnd.api+json"