Before you can get your WhatsApp Official Business Account green checkmark (which is now a blue checkmark as of mid-2024), your Meta Business account has to be verified.
This is often a straightforward process, and usually the most complicated step is uploading a few documents that prove your business is real if requested.
This Meta help doc outlines the process if you're interested in more details:
🌐 https://www.facebook.com/business/help/2058515294227817?id=180505742745347
The first step to get started is to click "Start verification" in your Business Manager’s Security Center: