The first thing to do to add the WhatsApp button to your posts is:
Prepare your post (text and image/video) and add it inside your page's post editor;
Once the post is ready, including text and image, you can go to the “add to your post” section by tapping the three dots;
Finally, click on WhatsApp and a button will automatically be placed that will redirect any user to your WhatsApp Business number;
Go to the configuration of your page, click on WhatsApp and finally add all the corresponding information.
Once all this is done, you will have your account configured, thus being able to enjoy the WhatsApp button in all your future publications.