When you've found a venue that looks like a good fit for a Threshow, you can reach out to the Host by selecting the green "Contact Host" button in the footer of their profile page. Here you can give them some background about your event and see if they're available for the dates/times that work for you.
The Host has 72 hours to get back to you. Once they reply you can freely message back and forth.
Once you've set up a Threshow with a Host, a group chat will be created for you, the Host, and anyone who has purchased a ticket to that event. This is a great way to inform the ticket-holders of any pertinent information about the show and create a sense of community beforehand.