Boarding pass or plane ticket itinerary??

Hello,

If i want to use the 72 hour exemption, can i use the plane ticket itinerary that you get through email as the outgoing 3rd ticket?
Its already paid for in full, its just that i dont have the actual boarding pass yet.
I dont know how to get the outgoing boarding pass and i usually dont book the seat ahead of time but mostly at the airport when im checking in.
Can i use the plane ticket itinerary as my outgoing plane ticket although it doesnt have a designated seat yet?