Lemon Squeezy's API at https://api.lemonsqueezy.com/v1 follows the JSON:API spec, so send Accept: application/vnd.api+json.
Create a key at app.lemonsqueezy.com. Send it as Authorization: Bearer <key>. In the examples it's written {{LEMONSQUEEZY_API_KEY}}.
Full reference: https://docs.lemonsqueezy.com/api
curl https://api.lemonsqueezy.com/v1/users/me \
-H "Accept: application/vnd.api+json" \
-H "Authorization: Bearer {{LEMONSQUEEZY_API_KEY}}"
curl "https://api.lemonsqueezy.com/v1/orders?page%5Bsize%5D=10" \
-H "Accept: application/vnd.api+json" \
-H "Authorization: Bearer {{LEMONSQUEEZY_API_KEY}}"
curl https://api.lemonsqueezy.com/v1/subscriptions \
-H "Accept: application/vnd.api+json" \
-H "Authorization: Bearer {{LEMONSQUEEZY_API_KEY}}"