You cannot create an addon domain if your dns hasn't already propagated to the server. So after you change the sites dns, you are trying to add you will need to wait 24-48 hours before attempting to create your addon domain.
The following does not imply any guarantee of it working sooner than the times stated.
This 24 - 48 hours is the official time period. However, I do suggest that you try after about 30 minutes. If it fails, then try again at one-hour intervals, and you may strike luck and get it sooner than expected.
Log in to your Cpanel
Locate the icon named addon domain and click on it.
There are four fields Cpanel asks for when creating an addon domain.
1. "New Domain Name:" You will put in the new domain name, for example, newdomain.com BUT do not put the http://www in the name box!
2. Username in box two will automatically appear as soon as you click in that box. MAKE A NOTE. If the name is more than 8 long, then delete any excess characters.
3. The document root will display in box 3 and will look similar to this. public_html/newdomain.com
4. "Password:" fill in a secure password, We strongly urge you to use the password generator button to create a password for you. PLEASE do not use anything less than 12 long and preferably 18 or somewhere between.
This is not only for your security but for the security of the other users of the server as well.
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