How do I post a pull request comment using BitBucket REST api 2.0?

I know it has been quite a long time since the question was asked, but for people coming to this post:

Bitbucket finally added a way to post comments using their 2.0 API. You check the documentation for more info.

And here is an example:

curl -X POST -d '{"content": { "raw": "your comment" }}' $URL

Unfortunately pull request comments are currently read-only in 2.0. We are definitely keen to finish that API, but these efforts have been rather under prioritized.

For now, 1.0 remains the only way to mutate PR comments.

Also see: https://answers.atlassian.com/questions/32977327/are-you-planning-on-offering-an-update-pull-request-comment-api