To do this, simply press and hold the WhatsApp icon on your home screen for about a second.
A small menu will then open, listing the camera and the four most recently used chats. With a simple touch, you will then be directly redirected to the corresponding conversation.
The trick works on iOS and Android devices. For Apple phones, you can also access the QR code scanner from the menu, or you can search for chats and create a new conversation.