To use WhatsApp WEB, you need to select the WhatsApp channel when purchasing a line.
To add a WhatsApp channel to an already created line, click New Channel in the line settings and select [WEB] WhatsApp.
Read more about buying the new line →
How to connect WhatsApp WEB
Go to the Selected company section in your Personal account and click on the desired line with WhatsApp.
Click the Connect button next to WhatsApp.
Scan the QR code displayed on your computer screen using the WhatsApp app on your mobile device.
Open the WhatsApp app, tap the three dots in the top right corner of the screen —Linked Devices — Link Device. 
You can send a QR code to the client for them to connect their phone to the line. To do this, click Share QR code — it will be copied to the clipboard.
The client will see a window with the QR code, which they can scan using the WhatsApp app.
After scanning the QR code, the line settings will show that the WhatsApp channel is connected.
To check if the channel is working, you can go to ChatApp Dialog and send a message to a contact through the [WEB] WhatsApp channel.
Recommendations
To securely store your chat history, we recommend setting up synchronization with Google Drive.
More details on how to set up synchronization with Google Drive →