Go to the 'Filtering > Domains' tab and add the following domains to
the whitelist, set to 'bypass filtering and authentication'
in WebTitan version 4.n
.apple.com
.itunes.com
.mzstatic.com
in WebTitan 5.n
apple.com
itunes.com
mzstatic.com
==> make sure Include Subdomains" is check
Then go to the 'Filtering > URL Keywords' tab and add to the whitelist
ax.init.itunes.apple.com
set this to 'bypass for domain only' and 'bypass from filtering and authentication'