Uniregistry:
1. Go to Account>Market
2. Click on "All Market Domains" in left sidebar
3. Click on the domain you want to change the lander on
4. Click Edit right underneath "Market Settings" on domain details page
5. Pick sales method (Offers only, Buy it Now, Offers and Bin)
6. Scroll down to Landing Page Status - turn it on
7. Select landing page type (Sales form, PPC, Affiliate)
8. Save
This is a new lander page and from the article, it appears this is being rolled out in a phased manner. So perhaps your account hasn't been updated to be able to use this new landers yet.. Maybe reaching out to their support can get you in front of the queue?