This is something you need right now. If you are starting to send messages to your users, you need to start getting the consent from your users.
Just add a form in your website where your users can opt-in for WhatsApp Messages.
The format of the form needs to be like this one. This is approved by WhatsApp.
You need to save this consents in your database and link it to your customer profile. We can help you creating this solution.