If you have an approved WhatsApp template that you submitted via Twilio, you can send that template using Courier.
Courier uses the Twilio API for WhatsApp as our delivery partner. This is the fastest path for getting your WhatsApp Business account approved. After signing up for this service, know the process can take multiple weeks on the WhatsApp side.
You can follow along with any of the Twilio WhatsApp QuickStarts to walk through signing up.
Once you've signed up through Twilio and had your WhatsApp Business account approved...
Submit your WhatsApp template for approval
For more on the process for creating a WhatsApp template and submitting it for approval through Twilio, see the Twilio doc: Send WhatsApp Notification Messages with Templates
Copy and paste your approved WhatsApp template into Courier
When your WhatsApp template is approved via the Twilio submission process, copy and paste the exact content into a WhatsApp channel in your Courier notification template.