To log on to e-Boks, you need a MitID.
You can get MitID for free through the MitID app or at a Danish Citizen Service Centre.
Read more about this process at https://www.mitid.dk/en-gb/get-started-with-mitid/.
If you have other questions about MitID, you can contact MitID Support.
You can find their contact information by following this link: https://www.mitid.dk/en-gb/help/mitid-support/.