Hi Carsten,
and welcome to our HiveMQ community! In case if you are using MQTT CLI client, you can connect to your HiveMQ Cloud cluster like this:
mqtt sub -i subscriber -t "#" -s -h 18b93f452b8445ba86ac0ec6d2228eb9.s2.eu.hivemq.cloud -p 8883 -u HiveUser1 -pw HiveUser1 -J -d
mqtt pub -i publisher -t "interesting/topic" -m "hello" -s -h 18b93f452b8445ba86ac0ec6d2228eb9.s2.eu.hivemq.cloud -p 8883 -u HiveUser1 -pw HiveUser1
The -s or --secure flag indicates to use TLS, the -u user and -pw password are needed to authorise the client to your cluster.
In case if your client device requires a server certificate, you can get one with openssl s_client tool as described in our FAQ here: Frequently Asked Questions
I hope this helps, if you are further questions please let me know.
Kind regards,
Dasha from HiveMQ Team