I have been put on the volunteer list for the second or third flight of a trip by on-line check-in and by an agent at the originating airport (different trips). Many agents do not want to deal with later flights in your itinerary or do not know they can do this and there are some who know they can but do not know how so you may have to hunt for one who is willing and knows how to do it. In all cases you need to provide an amount for them to enter. If the flight is not oversold most agents will refuse to do anything. Getting a prompt during on-line or kiosk check-in is hit or miss. Even if oversold, if the system believes all will be good, no prompt. If others volunteered before you and there are enough to cover the oversell, no prompt.