It is not easy to see how to comment, so here is the official change.org advice:

Adding a Comment

When signing a petition, you’ll have the option to leave a “Reason for Signing” - this is a public comment that allows you to share why you support the campaign! To do this, fill in the box above the “Sign this petition” button and you’re good to go!

If you’d like to post a comment on a petition that you’ve already signed, you can by following these steps while logged into your account:

Click on “My petitions” at the top of the page, and then select the “Signed” tab.
On your list of signed petitions, find the one you’d like to comment on, and click on it.
Scroll down the page and find the “Reasons of signing” section. There, you’ll be able to add your own reason or comment!