How can I add this extra part and what does it mean?
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This is called a subdomain and is typically done in the A record where your host your DNS records (quite often, the same place where you register the domain name)
You should be able to create new A records, simply enter the name of the subdomain and point it to the relevant IP address.
Then, your web server can be set up to redirect it to the relevant directory.
From memory, there are restrictions though such as you may not be able to start the subdomain name with numbers.