we can do this using value of settings.
# create domain list box in subservice using the following
<setting type=”list” id=”domain” element-type=”domain-name”>
<name>domain name</name>
</setting>
# in subservice again create a hidden field with type “list” as below. here use the value of settings
<setting type=”list” id=”subdomain” element-type=”string” value-of-setting=”domain” protected=”true” visibility=”hidden”>
<name>Subdomain list</name>
</setting>
# use the subdomain in a for loop in verification script and load it into a dyanamic drop down