Keep your existing database settings in config/config.php.
Add these optional/recommended keys under 'app':

'base_url' => 'https://YOUR-DOMAIN/YOUR-FOLDER',
'encryption_key' => 'a-long-random-secret-string',
'hmac_key' => 'another-long-random-secret-string',
'cron_key' => 'a-random-secret-for-web-cron',

Then add this top-level section after 'app':

'mail' => [
    'smtp_timeout' => 30,
    'max_per_run' => 50,
    'max_attempts' => 3,
    'retry_minutes' => 20,
],

V2 remains compatible with the old V1 config even if encryption_key/hmac_key/mail are missing,
but base_url is required for working unsubscribe links.