how to create new comment inside a comment using Box Rest API
there is Field "is_reply_comment"
but don't know how to set value for this field
To create a new comment in response to an existing comment, you would use the Create Comments API.
The body of the POST would follow the format below, where COMMENT_ID would be the id of the existing/parent comment:
{"item": {"type": "comment", "id": "COMMENT_ID"}, "message": "YOUR_MESSAGE"}
Here is an example using cURL:
curl https://api.box.com/2.0/comments \
-H "Authorization: Bearer ACCESS_TOKEN" \
-d '{"item": {"type": "comment", "id": "COMMENT_ID"}, "message": "YOUR_MESSAGE"}' \
-X POST