You need a CPR-number (civil registration number). It is unique to you and is used for everything from making doctor’s appointments to creating a bank account, getting paid and pay taxes. You should have a CPR number before you start working.
If you have a residence permit:
The municipality where you are going to stay has to register you in the Central Person Register (CPR) as having moved to Denmark. You will be registered in the CPR under the personal number you were given when you made your residence permit application. If you need help, you can contact your municipality (i.e. municipality of residence).
If you have not yet recived a residence permit:
You can contact the Danish Tax Agency to get an administrative CPR-number and tax card.
Find more information on this webpage:
When you are from Ukraine and get a job in Denmark (skat.dk)