How to mark a window in i3wm to catch it in Compton?

I've set the opacity for inactive windows in my Compton config. But sometimes I need to fix opacity for a window to keep it readability while it is inactive. I wanted to add a certain class to a focused window by pressing a certain key combination. So, now I have a key combination and I already specified opacity for windows of a certain class but I can't add a class to a focused window in my i3 config file. I was trying to search for a command to add a class to a window but found nothing but xprop (which isn't suitable for this operation). And now I think that I looking in the wrong direction. Maybe there's another way to mark a window somehow, to refer to it in a Compton config?