This gist seems to be not working anymore
Possible alternative solution https://unix.stackexchange.com/a/508728 ( not tested )
$> deb http://ftp.debian.org/debian jessie-backports main
-bash: deb: command not found
Open sources.list with sudo ( if available )
$> nano /etc/apt/sources.list
Paste the deb command in the end of file
deb http://ftp.debian.org/debian jessie-backports main
And them run
$> sudo apt-get update
Err:12 http://ftp.debian.org/debian jessie-backports InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010
Reading package lists... Done
W: GPG error: http://ftp.debian.org/debian jessie-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010
E: The repository 'http://ftp.debian.org/debian jessie-backports InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.