Segment's HTTP Tracking API at https://api.segment.io/v1 takes the same calls as its libraries: identify, track, page, group.
Use Basic auth with the source's write key as the username and an empty password — curl's -u key:, PostTaco's Auth tab. In the examples it's written {{SEGMENT_WRITE_KEY}}.
Full reference: https://segment.com/docs/connections/sources/catalog/libraries/server/http-api/
curl -X POST https://api.segment.io/v1/track \
-u {{SEGMENT_WRITE_KEY}}: \
-H "Content-Type: application/json" \
-d '{"userId": "user-123", "event": "Order Completed", "properties": {"revenue": 39.95}}'
curl -X POST https://api.segment.io/v1/identify \
-u {{SEGMENT_WRITE_KEY}}: \
-H "Content-Type: application/json" \
-d '{"userId": "user-123", "traits": {"email": "[email protected]", "plan": "pro"}}'