How do I create or edit the default page for new accounts in DirectAdmin?
As a reseller, you have the folder:
/home/RESELLERUSERNAME/domains/default/
Replace "RESELLERUSERNAME" with your reseller account's username and fill that "default" directory with anything you want new accounts you create to contain.
If you want to simply edit the default page, edit the existing "index.html" in the "default" directory.