Hi there! There's two ways you can do this.
This one's easier if you're on your desktop/laptop/a computer, basically. In classic editor, you'll see a tab on the right side that says "Categories" and you'll see a search bar looking thing that says "Add category." Type in Anti-Heroes
Another way you can do this is by editing your page through the same classic editor and add [[Category:Anti-Heroes]] all the way at the end of the page.
I personally wouldn't reccomend using the visual editor (because you can't edit templates that easily) unless it's for minor grammar issues, so don't use that editor.