You'll be able to communicate by WhatsApp by selecting the following option in the guest communication thread:
If you don't see this option in your account please contact firstname.lastname@example.org (The support team might need to activate the feature for your account).
Once selected, you might have to select the template you wish to send to the guest (if communication was not started yet):
For more information about templates don't hesitate to check our User Guide Understanding how to use Whatsapp Business Message Templates
You can then click the arrow to send the message/template and the guest will receive it on his/her WhatsApp account