Can I post a screenshot to verify a Safari extension?

Two comments, and a question.

• It is not possible to verify the authenticity of anything from a screenshot. Or, for that matter, a video, audio recording... etc. Videos of cute kittens playing basketball for example are no more "real" than a screenshot. Don't bother.
• Never download anything unless you initiated that action, you know what it is you're downloading, the reasons you want it, and how to get rid of it when you don't want it any more.

It’s about a safari extension it wants me to download.

Define "it".

In an editing window, click the icon of a landscape picture (second from right) to choose an image to place in a response here. Or, just drag and drop the image into the editing window.

But in general, if you didn't initiate any download, it's very, VERY unlikely it's anything you should install.