M&M3E - How to add a new User tag?

[Duggan]

So, I'm trying to help out with someone wanting a custom Extra for Shrinking in 3E. They've got a post in that forum with no answers as of yet. Looking through the code for Density, it looks like I would be adding a user tag with the Power Extra and checking for it in the Power. The catch is, I get an error indicating that I'm trying to use a user tag which is not defined. Is there a good way to add such a tag to the system? I've tried adding it under Tags and I've tried adding the tag directly into the .user file, but it still doesn't work.