HGG Community Forums

You would have to purchase the gene license by buying one from the Roving Gene Sale. Mushroom also occasionally shows up in the Gene Mod Treatment lab to be purchased with GMTs but those are usually only a couple of times per year occasions. Once you own the license, you can buy as many mushroom horses or breed as many from covers/straws as you want.

Edit: It's also possible to pay someone HBs at an exchange rate of 100k-200k per $1 when it pops up. They would add it to your horse by using your horse's ID number at checkout.