The RX-A720 looks to be a pretty typical HDMI switching AVR. If you haven't already, connect the AVR's HDMI Output to one of the television's HDMI inputs. Choose one of the AVR's unused HDMI inputs (it appears to have 5 total) and connect the Roku to it. Switch the television to the HDMI input connected to the AVR and the AVR to the HDMI input to which you connected the Roku and enjoy.
The remote for that AVR is pretty transparent--there are buttons for selecting the HDMI inputs marked 1 thru 5 in an area on the remote marked "HDMI". Usually there are buttons marked "BD/DVD", "Cable/Sat", "Game", "PC", "TV", etc and you have to know which input is which (or maybe that's just Onkyo