You can run the given code from command line of your Linux system:
php -r '$password = trim(shell_exec("openssl rand -base64 48")); if (strlen($password) === 64) {$hash = password_hash($password, PASSWORD_DEFAULT) . "\n"; echo "Insert as \"updater.secret\": ".$hash; echo "The plaintext value is: ".$password."\n";} else {echo "Could not execute OpenSSL.\n";};'
For me the output is:
Insert as "updater.secret": $2y$10$CiioqG06g7fZQUJYLgjnt.QLY.Us1YKSTIgPOuV82LH30hnWodL6.
The plaintext value is: AwjiEXiC1f9vJ6+bao3dCdmuqbmS+4V7wWFkszhpeqkIlyMnIEnpazDuWVPJ0iLx
the updater-secret goes into the config-file (that is the hashed value of the password),
and you need to keep the plaintext value and provide it for each update.