For Facebook, go to your page and locate the big blue button under the cover image of your page. Beware, the interface of Facebook changes regularly and could be different, but basically the information will give you a good idea of what to do.
If you do not have a button configured, it will be identified by "Add button", click on it and continue to Step B
If you already have a button it will be registered something other than "Add a button" such as Send Message or Call Now for example. Simply put the mouse on the button and a menu with options will be displayed then choose "Edit Button"
At this point, you may have the following dialog or you may be redirected directly to Step B below.
So click on Change (Your button) as outlined in red:
Be careful if you get a Facebook error, it is preferable to remove the button and then add a button instead of changing it. Eventually, you may have the Step B screen or the screen just above to change your button and set up the link at the box office. Facebook is not very intuitive at this level!
Step B, Choose Book Now from the list
And paste the link copied previously.