After you complete Mustard's Second Trial, Honey will announced that one of the Dojo Students will be permanently placed in the Master Dojo Kitchen. This character is none other than the Max Soup Chef, and he will be willing to help you make Max Soup whenever you so please. It will, however, cost you three Max Mushrooms in order to make a batch of Max Soup, which can then be used on a Pokemon for it to gain the ability to Gigantamax when in battle (if said Pokemon has a Gigantamax Form). In order to make Max Soup for Urshifu, you will need three Max Mushrooms and Sticky Honey.
Now that you know where to find the Max Soup Chef, Click Here to learn how to make Max Soup!
We have questions and answers related to this topic which may also help you: