FlatHub requires to verify my app: Install CoBang on Linux | Flathub
It tells me to:
Create a page at https://hoabinh.vn/.well-known/org.flathub.VerifiedApps.txt containing the following token
But hoabinh.vn is just a top-level domain, managed by Viet Nam Internet Network Information Center. My domain is quan.hoabinh.vn.