Chat via WhatsApp is enabled by default. You will find the settings in the Listovi Panel (User Panel). If the check box is not selected, chat via Whatsapp is available on your website. 

Now your user can check if they use WhatsApp in their user panel (checkbox “WhatsApp” text can be changed in the Listivo Panel > Translate & Rename).

You can now add a widget anywhere you wish on the listing page/user page. Important: If you cannot see the widget after adding it on mobile/tablet/desktop, please make sure you added it to the section that is visible on tablet or mobile, e.g. this way: