WSO2 APIM Dynamically Change Endpoint URL Synapse Sequence

in_sequence.xml

<sequence xmlns="http://ws.apache.org/ns/synapse" name="WSO2AM--Ext--In">
    <log level="custom">
        <property name="TRACE" value="Inside InSequence"/>
    </log>
    <log level="full"/>
    <property name="uri.var.sub_domain1" expression="get-property('system','environment.sub_domain1')"/>
    <property name="uri.var.port" expression="get-property('system','environment.port')"/>
    <log level="custom">
        <property name="HOST " expression="$ctx:uri.var.sub_domain1"/>
        <property name="PORT " expression="$ctx:uri.var.port"/>
    </log>
</sequence>