As the official WhatsApp site has just informed:
The multi-device beta is an opt-in program that gives you early access to try a new version of WhatsApp for Web, Desktop, and Portal. If you join the multi-device beta, you'll be able to use linked companion devices without the need to keep your phone connected.
The new feature is now available for all the accounts. To start using please follow the steps below:
1) join the beta program on your phone:
2)in WhatsApper for LiveChat open account settings, unlink the phone, switch to Multi-device mode, re-scan the QR-code and Save.
Please reach our support if you experience any issues.