To create a subdomain:
You can then add your subdomain into the domain field and select submit, doing this will create the subdomain inside the same public/html as the doamin. You're also able to change the Document Root so that you can direct the subdomain to the folder of your choosing by unchecking the share document root box.
What is the document root?
The document root is the folder which the subdirectory will use, for example, you may have a directory within your web space located within public_html called support (i.e. public_html/support), you could point your subdomain to this location by adding public_html/support in the Document Root field.