If anyone is struggling with this after the new update to the platform and doesn't want to spend an hour trying to find someone in support to help here is where you now update the license number.
1. Go to the listings
2. Click on the listing, so you are in the 'Listing Editor' screen
3. Click on the three lines to the right of 'Arrival Guide'.
4. Click on 'Regulations' to add the Registration Number.
Hope this helps.