How do I get my dog officially registered as an Emotional Support or Service Dog?

I have a form of autism and many other mental illnesses. I was wondering how I could get my dog registered as an ESA without just buying a vest online which is something I don't think is right. Please and thank you!
Update: Sorry to ask such a sensitive question guys. I hope I didn't hurt your feelings while asking this triggering question. (ahem sarcasm) Get a life.
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