Home Assistant Community Add-on: Nginx Proxy Manager

You mean this part?

https://docs.nextcloud.com/server/13/admin_manual/configuration_server/reverse_proxy_configuration.html#overwrite-parameters

  'trusted_proxies'   => ['10.0.0.1'],
  'overwritehost'     => 'ssl-proxy.tld',
  'overwriteprotocol' => 'https',
  'overwritewebroot'  => '/domain.tld/nextcloud',
  'overwritecondaddr' => '^10\.0\.0\.1$',

Read full topic