On your Facebook page, edit your action button as shown below.
These are the steps for those on the New Version. If you are on the old version or the steps here have not helped - please find steps here!
If your page has a 'Posts' button then you are using the New version.
For steps on using the Classic Version, please see this article
Step 1
Click the 3 dots then Edit Action Button
Step 2
Click Contact Us option in the list.
NB Please do not click Book Now as this is not supported by Facebook in the New Pages. Facebook Uk have not released an update on when this will be available in the UK.
Step 3
In the Action Button box, paste your Booking Link
Your booking link can be found In Online booking.