To be explicit for anyone else that comes across this post:
curl -XPUT 'http://host:port/_license?acknowledge=true' -d @license.json
The above cURL request is just a PUT request that sends the license.json file as the body of the request. As the file extension implies, the file contains a JSON data structure. @license.json is used instead of license.json as a way to tip off the cURL command that it needs to dump the contents of that arbitrarily named file as the body of the request.