WhatsApp message templates can be sent proactively. This means you can send them to contacts who have never sent you a WhatsApp message in the past.
However, you must obtain and keep a record of your user's consent before sending them a WhatsApp message for the first time.
For more information on this, here's Facebook's official guide on how to obtain opt-in
Please note the following paragraph from the official guide:
People can block or report businesses and our systems will rate limit businesses if the business’ quality is low for a sustained period of time. We may also reactively evaluate a business’ opt-in flows, including user feedback, to flag policy violations and develop additional types of enforcement over time.