Paddle Billing's API runs on https://sandbox-api.paddle.com for testing and https://api.paddle.com for live.
Create a key at Paddle → Developer tools → Authentication. Send it as Authorization: Bearer <key>. In the examples it's written {{PADDLE_API_KEY}}.
Full reference: https://developer.paddle.com/api-reference/overview
curl https://sandbox-api.paddle.com/products \
-H "Authorization: Bearer {{PADDLE_API_KEY}}"
curl https://sandbox-api.paddle.com/customers \
-H "Authorization: Bearer {{PADDLE_API_KEY}}"
curl https://sandbox-api.paddle.com/transactions \
-H "Authorization: Bearer {{PADDLE_API_KEY}}"