The index.html is located in the domain root, which should be something like this if you used the defaults:
And when you set up the FTP account, set the username, password and account type, then select Use Domain directory and select the new domain.
Code:
/var/sentora/hostdata/clientname/public_html/domain_com/index.html
And when you set up the FTP account, set the username, password and account type, then select Use Domain directory and select the new domain.