I do not believe there is any way to easily fix this problem without 000webhost's intervention at the DNS level since either an alias for 'www' or a messier wildcard '*' record would be the only straightfoward way to fix it.
is not resolving for you (I have not tested it myself), then they are very likely not providing either of those DNS record types.
If that is the case, 000webhost do provide a 'www' alias when you create create a domain
with them (including their own provided ones) but not subdomains. After checking a few other top-level internet domains, this seems much more common than providing an automatic 'www' alias for subdomains.
Given low level DNS management is not part of 000webhost's free offering, likely the only way to fix it here is by duplicating your site (adding a www.sub.domain.com
, in addition to your existing sub.domain.com) or by upgrading your account and asking for additional DNS services.