When you have the MIDI Multi Recording display open, just touch the instrument icon of the channel that you want to record.
This will open a window which allows you to select the keyboard part (R1 for example) that will be used for the recording. Touch the part that you want to use in the list, and touch 'OK' to actually select it. The channel number icon will change from green to red indicating that it is armed for recording.
You can arm more than one channel for recording at the same time, but if you do this, you will only be able use each specific part in one of the armed channels.
There is information about MIDI Multi Recording in the section "Recording Each Channel Individually (Realtime Recording)" starting on page 69 of the 'Genos Reference Manual'. This is different to the 'Genos Owner's Manual' and is not supplied with the keyboard. However you can download a PDF copy from the Yamaha Downloads web site. Here is a link to the Genos assets page containing the manual on the UK site which should work for you even if you are not in the UK, or you can find the page on your local Yamaha site...
https://uk.yamaha.com/en/products/musical_instruments/keyboards/arranger_workstations/genos/downloads.html#product-tabsRegards
Ian