While we refer to the process as "adding a Subdomain", the actual method of doing this is called "adding an A record".

Adding an A record with GoDaddy
 

1. Log in to your GoDaddy account, and go to the DNS management page.
    
2. On the DNS Management page, at the bottom of the Records section, click Add and select A from the menu.
    
3. Complete the other fields:
   • Host Name - Enter the host name the A record links to. This will be booking or bookings depending on your personal preference.
   • Points to IP Address - Enter the BoothBook IP address which is 77.68.74.83
   • TTL - Select how long the setup should take. This is usually 14400 or 30 minutes
      
4. Click Save.

For help on adding an A Record for domain registered at another company, take a look at GoDaddy's article .