It's true, using urine to make ammonia has been known since the Egyptians. All you need do is let it sit, and it changes on its own. This is why cat litterboxes smell like ammonia when you let them sit.
However, as a medicinal, I wouldnt recommend it. What it was used for in those days was a mordant for cloth dyeing, and as an ingredient in wood stains.
Also, there is a way to extract saltpetre from urine, basically doing the same thing, but it takes a loooong time.