Custom categories to track your spending are set up on the Spending tab. Once created you can include them as budgeting goals too.
Head to the Transactions tab.
Select the icon to the left of one of your transactions and select Create a category.
Type in the name you'd like to set for the category and choose an icon you'd like to use. Select Save.
Note: You'll need to manually add transaction to your new category in order for them to count towards your spending totals.