You can add a live chat to your site in your site editor in the "Contact + live chat" section. By default your site already includes a 1-click WhatsApp button so that your visitors can easily contact you. But if you prefer you can also use Tawk.to live chat or the Facebook chat plugin instead of WhatsApp. All the options and guides to configure these are available in the live chat preferences page in your site editor (here is also the direct link if you prefer at https://builder.portmoni.com/builder/websites/update/#contact )
Related questions:
> I need a live chat
> Can I have an online chat feature
Old answer (do not use)
Unfortunately, at the moment this feature is not available, please accept my apologies about this. However, your site already includes a 1-click WhatsApp button so that your visitors can easily contact you. The advantage of whatsapp over live chats is also that you can still answer your visitors after they have left your site (with traditional live chat, as soon as the visitors leave the chat, there is no way to answer or restart a conversation). Does that make sense?