Neet help in using filters to bypass the Checkout page

I would like to have in my site the old functionality where we were able to skip the Checkout page when using the “Buy Now” button. Since I am not interested in using “Add to cart”, I would like to add a filter on edd_purchase_link_defaults to make all buttons go directly to Paypal. I read somewhere else that we need to “add a filter on “edd_purchase_link_defaults” and overwrite “direct” parameter on “true” so always all buttons will directly send to PayPal no matter what is selected at “Purchase button behavior” option.”

All that I know is that edd_purchase_link_defaults is located in ?/easy-digital-downloads/includes/template-functions.php… line 66.

… How can I “add a filter” to achieve my goal? Where should I place that code?