How can I highlight text on a screenshot using Markup in macOS?

When I take a screenshot in macOS Mojave, I get a handy screen with Markup tools to edit the screenshot. But I’ve never been able to get the “highlight” feature to work. I want to highlight text on a page in yellow, similar to a highlighter pen. It looks like it should do it, but the highlighter tool is always disabled. Nothing I do seems to enable it. Does this tool work? How can I use it?