Have a many to many relationship ship between the businesses and categories.
(One businesses can have multiple categories and a categorie can belong to multiple businesses)
Have a screen that creates the business, after that have a screenwith a multi select, having a list of categories.
Then the field I choose to update is the “Service Categories” (which is the relationship field), then I select “Add”, “Current Service Category” (See Image)
Changed one thing tho, I created a many to many relationship between the users and Businesses (A user can have multiple businesses in real life anyways right?)
(Ofcourse you can disable this using your UI, if you want a user to be able to create only one business).
If you would like more help, or a call let me know!