I solved it as follows:
<Call class="java.lang.System" name="setProperty">
<Arg>javax.net.ssl.trustStore</Arg>
<Arg>trust.jks</Arg>
</Call>
<Call class="java.lang.System" name="setProperty">
<Arg>javax.net.ssl.trustStorePassword</Arg>
<Arg>xxxx</Arg>
</Call>