Hi @Dushan0 click on the listing you want to add the co-host to, go to edit, and towards the top of the page you'll see a list of actions see below (they run horizontally along the page actually!)
You might see co-host slightly hidden from view, so just click along, then click on co-host, then the "invite a friend" button, and add the email address of the person you want to co-host for you!