To add an action button to your Facebook Page, you'll need to be an admin, editor, moderator or advertiser of the Facebook page.
If it is the case, log in to your page and press on "add button".
Then, select the "shop now" option.
Then, just add the URL of your website where you display your bookingkit widgets.
That's it! Your customers now can access your bookingkit webshop directly from Facebook.