Here's a guide to how you can activate a waiting list already before the ticket sales starts. You can use this for pre-orders on ticket sales or for a simple audience curation process enabling you to remove rejected users from the waiting list.
When you set up a ticket set the available amount to 0 and activate the waiting list option. The waiting list will then be available to your website visitors right away. At a later time, when you are ready to actually sell the tickets you can set the actual number of available tickets for your event. Note, that you need to set the price right when setting up the tickets, as the price cannot be changed for at tickets, once people have signed up to a waiting list for it.
You can use this option to track interest in a ticket type before you start actually selling the tickets, as a pre-order option. This can be useful e.g. for events that knows that they need to sell at specific amount of tickets to make ends meet. While this method is good for tracking interest in your event or a specific ticket type, it does not guarantee that the tickets will actually be sold, but it increases the likelihood for it to happen.
You can also use this option as a simple way of audience curation. To do this, simply review people on the waiting list and and remove any rejected users from the list before setting the actual amount of available tickets. This way the rejected users will not receive any reservation link email.