I fixed it. Just forgot to precise the protocol at the beginning of the url.
The type of connection required is specified by the scheme of the connection URL set in the ClientOptions struct, for example:
tcp://iot.eclipse.org:1883 - connect to iot.eclipse.org on port 1883 using plain TCP ws://iot.eclipse.org:1883 - connect to iot.eclipse.org on port 1883 using WebSockets tls://iot.eclipse.org:8883 - connect to iot.eclipse.org on port 8883 using TLS (ssl:// and tcps:// are synonyms for tls://)
That works :-)