To be able to test/use this api you need Access Token, Phone number ID and Business ID. For that tou need to register as a Meta Developer. You can register here https://developers.facebook.com/docs/development/register
Then create an application here https://developers.facebook.com/docs/development/create-an-app and configre it to enable access to WhatsApp Business Cloud API.
You can manage your apps here https://developers.facebook.com/apps/
From Whatsapp Developer Dashboard you can try and send a test message to your phone number. to be sure that everything is working fine before you start using this api.
When all the above is done you can start using this api.
Head over to HOWTO file