To take a screenshot on a Mac computer, please see this article.
What is a Screenshot and Why is it Useful?
A screenshot is a way of taking a snapshot, or picture, of your computer screen. Something that you see on your computer we may not be able to see on ours, so if you can grab a screenshot it will allow us to better understand what the problem is when troubleshooting.
Take a screenshot of your whole screen:
There are other downloadable programs available for use on both the Mac and PC to quickly capture and share a picture of your screen. These are helpful programs that allows you to capture your whole screen or just a part of it. They give you the screenshot URL for easy inclusion into support ticket: