If you are not already in Norway, you can apply for a visitor's visa (Schengen visa), which makes it possible for you to travel to Norway to apply for a residence permit according to the Brexit regulations from Norway.

When you apply for a visitor's visa, you must submit documentation of your family relationship with the British citizen and documentation which shows that he or she is an employee, self-employed, has sufficient funds or is a student in Norway, and that he or she was resident in Norway at the end of 31 December 2020. You normally have the right to a visa, a right to a prioritized processing of your application and you do not have to pay a fee.