Amplitude's HTTP V2 API at https://api2.amplitude.com/2/httpapi ingests events. The project API key goes in the JSON body.
Copy your project's API key from Settings → Projects in Amplitude. In the examples it's written {{AMPLITUDE_API_KEY}}.
Full reference: https://amplitude.com/docs/apis/analytics/http-v2
curl -X POST https://api2.amplitude.com/2/httpapi \
-H "Content-Type: application/json" \
-d '{"api_key": "{{AMPLITUDE_API_KEY}}", "events": [{"user_id": "user-123", "event_type": "Signed Up"}]}'