How can I hold an appointment without knowing who the subject is?

The Scheduler application allows holding of visits without a subject.   When scheduling a new appointment, there is an option to "Hold appointment without subject" .  This will allow you to hold the appointment.  

When subject information becomes available, a user can go to the Hold appointment and swap in the desired subject by clicking on the "Switch Subject" link.  

Similarly, a scheduled appointment can be changed into a hold appointment by using the "remove subject" link on the appointment pop up screen.