There was a targeted ad campaign for 10 tenders for $10 on Facebook and Instagram (screenshot if the link doesn't work). Note that this completely skips the targeted nature of this deal - using this deal without being targeted could be considered unethical.
The deal link only works on a phone with the KFC app installed.
As this was meant to be targeted, there is no official kfcau.app.link URL for this offer.
Ok, I finally got it working just had to update Firefox haven't updated it in 1.5 years. After that I could see the "Open IN" in the Firefox toolbar menu.
I also got it worked by installing the "Open Link WIth" App in the play-store and sending the URL to that app.