Skyscanner's Travel API at https://partners.api.skyscanner.net/apiservices/v3 is for approved partners. Flight search is two steps: create a live search, then poll it with the returned sessionToken.
Partners get a key from Skyscanner and send it as the x-api-key header. In the examples it's written {{SKYSCANNER_API_KEY}}.
Full reference: https://developers.skyscanner.net/docs/intro
curl -X POST https://partners.api.skyscanner.net/apiservices/v3/flights/live/search/create \
-H "x-api-key: {{SKYSCANNER_API_KEY}}" \
-H "Content-Type: application/json" \
-d '{"query": {"market": "US", "locale": "en-US", "currency": "USD", "queryLegs": [{"originPlaceId": {"iata": "JFK"}, "destinationPlaceId": {"iata": "LHR"}, "date": {"year": 2026, "month": 11, "day": 15}}], "adults": 1, "cabinClass": "CABIN_CLASS_ECONOMY"}}'
curl -X POST https://partners.api.skyscanner.net/apiservices/v3/flights/live/search/poll/{{SESSION_TOKEN}} \
-H "x-api-key: {{SKYSCANNER_API_KEY}}"