You will find the Placeholder Registrations settings in the Division settings of your program. Once you select Allow Placeholder Registrations for this Division, you will need add a Title and any questions you want answered during the participant registration.
Important: if you title the placeholder "Waitlist", it is going to function like a traditional waitlist where the option only populates if the session is full.
If you name it anything other than waitlist, it will allow members to begin the registration process and be put on a placeholder list. The most common use for this are grant funded scholarships where you might need to sign extra paperwork or provide extra documentation in person.