Here are the instructions to create the subdomain if your domain is hosted at Register.com
1. Login to your Register.com account.
2. Under the My Domains and Services area in the middle of the screen, click on the URL you want to add the CNAME record to.
3. Scroll down the page and under Advanced Technical Settings click on Edit Domain Alias (CNAME).
4. Add a new domain alias as 'm' and have it point to mobile.dudasite.com. The final record should look like "m.yoursitename.com points to mobile.dudasite.com"
5. Click Continue.
6. Click Continue to confirm.