After ddclient has been installed, the file /etc/ddclient.conf has to be configured as follows:
protocol=dyndns2 server=dyndns.inwx.com login=your-dyndns-login password=your-dyndns-passwortyour.dyndns-hostname.com
protocol=dyndns2 server=dyndns.inwx.com login=your-dyndns-login password=your-dyndns-passwortyour.dyndns-hostname.com