If you have an active eBay Store subscription, your Store Categories will be automatically available as options.
However, if there is somewhere else you'd like to send your visitors, follow the steps below:
Right-click on a link from eBay and select Copy Link Address
In the categories section, hover over a category and click the edit button:
In the pop-up window, paste your link:
After saving and re-applying the design to your listings, your category button will use your custom eBay link: