Simply connect an HDMI cable between the Nucleus One and a HDMI port on the Marantz. Go to Roon’s 
> Audio screen and enable the relevant port in the Roon Server section. On the Marantz switch the input source to the HDMI port that the cable is connected to.
Open up the Roon Device Setup screen on the relevant port; for example:
In the Roon Device Setup screen, you want to scroll down and open the Advanced Settings section (note: no scrollbars are shown, but it is a scrollable window). Then scroll down further to show the Channel layout setting and set this for multichannel working. For my Denon AVR this is 7.1:
See also: