Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

The new rules for apt-keys need some attentions:

  1. download an convert the .key file (Release.key) to .gpg
  2. create a new list file in /etc/apt/source.list.d/
  3. After all reload source list
wget -O-  http://download.sample.com/Release.key |gpg --dearmor | tee /usr/share/keyrings/sample-keyring.gpg
echo "deb [signed-by=/usr/share/keyrings/sample-archive-keyring.gpg] https://download.sample.com/ ./" | sudo tee /etc/apt/sources.list.d/sample.list
apt-get update