Focusing in the dark must be done beforehand and at a distance called the hyperfocal distance. It is the distance specific to a particular aperture and focal length used from which everything half the distance towards the camera and from which everything to infinity is in acceptably sharp focus.

Take a look at this table:

https://www.photopills.com/calculators/hyperfocal-table

If you use a large aperture as you should when photographing the night sky and you have set your objective at a short focal length to maximize what's visible in the frame with your settings being 20 mm and f/4, for instance, the hyperfocal distance is about 5 meters if you use a crop sensor Nikon. Use the table to find the right values for your camera and objective and exposure settings.

The focus at that distance or further (further in case there is nothing interesting in the foreground). Use a flashlight or the headlights of your car, if that's convenient to focus manually or with auto-focus.

Make sure your lens is on manual focus so as to keep the focus constant.

Use a large aperture and an exposure time of about 10-15 seconds (if more than that the stars will leave a trail). Set ISO at 1600. Finetune the settings to get the best possible results. Sometimes you need higher ISO. But aurora borealis is bright, so maybe not here.