Adyen's Checkout API v72 lives at https://checkout-test.adyen.com/v72 for testing; live URLs include your account's prefix. Amounts are in minor units: 1000 = €10.00.
Create an API credential under Developers → API credentials in the Adyen Customer Area and send its key as the X-API-Key header. In the examples it's written {{ADYEN_API_KEY}}, with your {{MERCHANT_ACCOUNT}}.
Full reference: https://docs.adyen.com/api-explorer/Checkout/latest/overview
curl -X POST https://checkout-test.adyen.com/v72/paymentMethods \
-H "X-API-Key: {{ADYEN_API_KEY}}" \
-H "Content-Type: application/json" \
-d '{"merchantAccount": "{{MERCHANT_ACCOUNT}}", "countryCode": "NL", "amount": {"currency": "EUR", "value": 1000}}'
curl -X POST https://checkout-test.adyen.com/v72/sessions \
-H "X-API-Key: {{ADYEN_API_KEY}}" \
-H "Content-Type: application/json" \
-d '{"merchantAccount": "{{MERCHANT_ACCOUNT}}", "amount": {"currency": "EUR", "value": 1000}, "reference": "posttaco-order-1", "returnUrl": "https://posttaco.dev/", "countryCode": "NL"}'