in the normal case of a shared server, there would be no A entry for *, right?
so there is step number 5, which is to create an A entry in the domain's DNS zone, in the same manner as for www and mail.
Thank you for your help.
I think I got it covered now!
Final procedure for anybody searching in future the same thing on the forum :
1- create a site standard normally, domain.com
2- inside that site, create a /subdomain/ subfolder
3- create a new site on ISPC, with domain subdomain.domain.com
4- setup an "L" redirect to folder /subdomain/ (typed like that exactly)
5- create an A-type record associating subdomain to the IP in the DNS Zone of domain, in the same manner as mail or www.