Before you can message a contact using WhatsApp, you need to receive permission from your contact. (See here)
Thereafter, you can indicate that the contact has opted-in by doing so on a per contact basis or per group basis on the CommzGate portal.
On a per contact basis, click on 'Edit' for the contact and check the 'Opt In for WhatsApp' option and then click 'Save'
For per Group basis, select the contact group and click the 'Opt In for WhatsApp' button.
More info about WhatsApp opt-in rules can be found here: