It's not possible to comment anonymously on Crowdfunder projects.
Adding a comment
After pledging on a project, you will have the option to add a comment and show your support for that project. Comments are always displayed with your username.
Anonymous pledges
If you have chosen to pledge anonymously, you will not be able to add a comment. If you'd like to add a comment to your pledge, you'll first need to swap your pledge over to display your username publicly.
Removing your comment
You can remove your comment by following this guide.