How to setup Single Instance of WhatsApp Business API Client

The WhatsApp Web Business Tool is deprecated since v2.29.1 You can only interact with your instance via REST API. You can leverage our Postman collection for this https://github.com/fbsamples/WhatsApp-Business-API-Postman-Collection

Sounds like you are ready to proceed with Step 6

https://developers.facebook.com/docs/whatsapp/installation/dev-single-instance#step-6--login-and-authenticate