If a title is not currently available, you'll see the options to place a hold. You can place a hold from within a sample, from the title details page, or directly from your search results. 

To learn more about the wait time and your place in line, tap the clock icon. Tap once to place a hold and automatically be added to the wait list. Once you've added yourself to the wait list, you'll see your estimated wait time and the date the hold was placed. 

Track and manage your holds on the shelf tab in Sora.