Unfortunately, HubSpot doesn't let you auto-subscribe contacts to WhatsApp communications. You have no direct access to the subscription type in workflows and you don't even see the subscription in the settings.
The option you have is to go into contact's profile and inspect all their subscriptions there. You will see WhatsApp and you'll be able to subscribe the contact manually.
(don't mind the German language)
Why don't we have an option to auto-subscribe the contact to WhatsApp Notifications if they submitted a form with their phone (WhatsApp) number and agreed to receive notifications via WhatsApp? When we collect consent from our contacts by ticking a checkbox or by clicking a button, we expect the contacts to be auto subscribed to WA-notifications.
P.S. I am talking about a situation when a contact doesn't know your WA number and cannot contact you first (as described here), so it is way better for both sides when a contact submits a form and gives their phone number and asks to be contacted per WhatsApp. The rest should be handled by HubSpot.
Added: There's currently an option to mass opt-in contacts for WhatsApp communications in your Contacts view. This is currently in Beta so make sure you add this function to your HS portal in Product Updates. STILL this is a manual process that is not convenient when you actively use WhatsApp for communications with your contacts.