The code is likely not applicable to the ocean cruise you are trying to book. Appears to apply to Mississippi, Great Lakes and Antarctica cruises based on the link provided above.
https://www.viking.com/promotions/us/en/portal/LD23.html
For the World Cruises… Free business class airfare. Complimentary visa service and silver spirits package. Shipboard credit and ground transfers, to name a few.
Offer codes can be used online and by phone. Go to Viking.com > Select the Cruise > Select ‘Price & Build’ > Select a Date> and right above the Date options you’ll see a box to input the offer code > Input the code & Select ‘Apply’
Hope this helps.