Getting a unique address for a mother-in-law apartment

[Chris Sturniolo]

I am just getting started in Denver, CO with a house hack.  I found a good property with a mother-in-law in the basement with a separate entrance.  I talked to my mailman, and he said I can't simply add a second box for renters to be able to receive mail.

Do I need to somehow set this up with the city?  If so, are there any requirements that need to be in place to qualify for the other address?  

Any help or insight is greatly appreciated!