You have to be a VIP and select it as one of your perks. Once that is done, search in sales for what you want and then press save search. For example, RCs under 80 passes. You have to select a max price otherwise the automatic purchase won't work. So you cant just save a search with for example any RC you don't have without adding a max price.
On the sales page it will come up with a button to manage your purchases automatically, here you have to activate the automatic purchase by adding a max number of horses to buy.

Thank you for answering