You can add a 'Tickets' button to your Facebook event by entering your Bookwhen URL into the 'Ticket URL' field.
Please note this is only an option if you are using a Facebook business profile, not a personal profile.
You can choose to either link to your public schedule page where all your events are listed, or link to a specific event page. More info on how to find the right URL here.
This is what the 'Tickets' button will look like once implemented and you've saved your event: