If you drive a car with foreign number plates, you must – within 30 days – register your car in Denmark and have Danish number plates for your car.
You do so via www.skat.dk/motorregistret.
In case you have no CPR number or no NemID (secure login site for online activities), you can send an e-mail to motorregister@skat.dk in which you ask for the performance of a duty calculation concerning your vehicle. You can read more about this rule on skat.dk.