Add a subdomain to my Windows Hosting account
Subdomains let you create memorable addresses and easy ways to access areas of your website. For example, you could add a subdomain for pictures on your site called "pics" that is accessible through the URL pics.example.com in addition to www.example.com/pics.
- Go to your GoDaddy product page.
 - Select Web Hosting, and next to the Windows Hosting account you want to use, select Manage.
 - In the Dashboard for the account, select Plesk Admin.
 - In the Plesk Websites & Domains page, select Add Subdomain.
 - Enter the Subdomain name you want to use.
    	
Note: You can't use a subdomain of a domain name that's not hosted on this account.
 - Select OK.
 
More info
- Here's the Plesk documentation on adding subdomains.