In the Finder, you can select a file and choose Tags... from the File menu or contextual menu. It will show a little dialog that allows you to select or type tag names, and you can press Enter to apply them to the selected file.
Before Mac OS Mojave, I was able to assign a keyboard shortcut to this menu item (using a method like this, although I'm using Keyboard Maestro).
But with Mojave, Tags... is no longer a standard menu item, so it can't be accessed this way. Using an ellipsis character instead of three periods doesn't help. Is there a way to do this now? I tried scripting System Events, but couldn't get it working.