As a handling editor of a manuscript you can post comments either to the authors or to the reviewers, once the first round of comments is completed, i.e. interactive review forum has been activated and the authors have responded to the reviewers' comments.
That means, after the authors had a chance to reply to reviewers' comments. In that stage, you can click on "Add Comment" below any of the existing comments.
Once you have finished commenting, make sure to submit your comments.
Once you have submitted all comments, the History of the manuscript is updated automatically and email notifications are sent to authors and the reviewer.