It's easy to add comments to a milestone in the operations section of the platform.
To do so, first go to the operations part of the job (contract has to be signed for this section to be enabled) by clicking the highlighted icon in the top navigation bar.
Once in the operations section, ensure that you have selected the right milestone by clicking the appropriate milestone tab. Immediately below the milestone tab label, you'll see a section that says "Comments". Clicking on "Comments" will reveal a new section called "Comment History", which will show historical comments. You can also click on "+ New Comment" to add a new comment.
Note that the "Add Comment" section to the right of Documents and Upload is for comments specific to a task, not a milestone. For more detailed information on adding comments to a task, click here.