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To create a comment, call the POST /comments API with the message of the comment, as well as the ID of the file to leave the comment on. 
curl -i -X POST "https://api.box.com/2.0/comments" \
     -H "authorization: Bearer <ACCESS_TOKEN>" \
     -H "content-type: application/json" \
     -d '{
       "message": "Review completed!",
       "item": {
         "type": "file",
         "id": 426436
       }
     }'
A comment’s message can also mentions users using the @ sign. To do so, add the string @[userid:name] anywhere within the message. The user_id is the target user’s ID, where the name can be any custom phrase. In the Box UI this name will link to the user’s profile. Then, pass this string as the tagged_message instead of the message.