It depends on which control panel is used on the server where you created your email address.
If you are on a server with the Confixx Control Panel, you can access webmail by appending /roundcube to the end of the server URL. For example, if you have hosting on the server 124.hosttech.eu, you can reach the webmail via the URL https://124.hosttech.eu/roundcube
If you are on a server with Plesk Control Panel, then you can reach the webmail via https://webmail.yourdomain.eu, where of course you have to replace “yourdomain.eu” with your own domain. It is important that the corresponding DNS record (type A for webmail.deinedomain.eu) also refers to the IP address of the server. You can find the IP address on your access data sheet.