You can see the MySQL address (hostname) by logging into My Zone control panel in in the Webhosting management by navigating to Databases > MySQL/MariaDB.
The MySQL address (hostname) takes the form dxxxxx.mysql.zonevs.eu
NOTE: It is NOT RECOMMENDED to use “localhost” or any other hostname as MySQL server address, as the MySQL server address is different for each domain and your MySQL database may be moved to another server as the load increases.