Feature.fm's "ffm.to" domains are whitelisted by YouTube for YouTube Cards.
What are YouTube cards?
YouTube Cards are interactive “panels” that slide in and out when a video is playing, encouraging further interaction by viewers.
Link directly to your Smart Links in your YouTube videos
How to add your "ffm.to" links in YouTube cards
1. Go to "My Videos" in the Creator Studio
2. Click "Edit" on the video you want to add a card for
3. Go to the "Cards" tab and then click "Add card." Select the "Link" option to include your Feature.fm Smart Link
4. Simply paste your Feature.fm Smart Link and click next.
Note that any URL that has "ffm.to" is already approved by YouTube, including sub-domain URLs (something.ffm.to/somethingelse). You will not be able to include a full domain that does not include ffm.to unless you've already approved it with YouTube.
5. Customize your card's details and click "Create Card"
That's it! Now your card is added to your video and viewers can click to your Feature.fm Smart Links directly from your YouTube videos!