I am not certain of the wiring that you have but a DC voltmeter can hook up directly or through a relay that is switched separately or switched by the bike's ignition. The voltmeter itself should have an impedance high enough that it does not present a load to the battery, but I am inclined to add a fuse to both the + and the - terminals of the voltmeter especially if you do not know the rejection status of the meter the fuse can be a 1 Amp, just for your peace of mind even in case you cut or short the leads at some time in the future. Better to be over cautious.
If the leads come from the battery there is a great potential and greater amp flow if the added wiring is unprotected.
Good day! I wasn't aware Honda offered an OEM volt meter. If you have one that is installed on the right panel with key, there is two possiblelocations of the connector. One, right below to the left of the panel in a rubber boot. If not there remove the right pocket and feel up under the right turn signal for the pigtail. I believe they are plugging in where the heated grips would normaly. Good luck...!