The quickest way is to use the Domain Manager within the panel.
Go to Domains->Manage Domain->Add New Domain/Sub Domain.
Then once there just enter the new subdomain.mydomainname.com in the "Domain to host:" box, select any extra options you need and click on "Fully Host this Domain Now!".
This will add the required dns entries, and create a folder which you will see when you login to the FTP server called subdomain.mydomainname.com.
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