You can't just put an HTML anchor link in the command. Remove this and put the URL at the end of the command:
curl -X POST
-H 'Content-Type: application/json'
-d '{"beacons": [{"id": "a", "location": [10, 0]}, {"id": "b", "location": [0, 10]}, {"id": "c", "location": [0, 0]} ]}'
http://tp-api.herokuapp.com/beacons/
One liner:
curl -X POST -H 'Content-Type: application/json' -d '{"beacons": [{"id": "a", "location": [10, 0]}, {"id": "b", "location": [0, 10]}, {"id": "c", "location": [0, 0]} ]}' http://tp-api.herokuapp.com/beacons/