Can not figure out how to add comments to PENDING review

I have been reading and trying out the APIs in different ways, but I can’t seem to be able to figure out how to add comments to a PENDING review.

I can start a review using

POST /repos/{owner}/{repo}/pulls/{pull_number}/reviews

and since not passing the event it is created in PENDING state properly.

Then I try to use

POST /repos/{owner}/{repo}/pulls/{pull_number}/comments

to add comments to this review, but it seems like it is trying to create a new review, as it returns
"user_id can only have one pending review per pull request"

So what is the proper way of adding comments to a PENDING review, which I would submit later using the Github API v3?