First retrieve the address value using the expression "//properties/property/@value". Then set the To address of the header mediator and send the message.
<property name="address" expression="//properties/property/@value"/>
<header name="To" expression="get-property('address')"/>
<payloadFactory>
<format>
<p:echoInt xmlns:p="http://echo.services.core.carbon.wso2.org">
<in xmlns="">$1</in>
</p:echoInt>
</format>
<args>
<arg value="1"/>
</args>
</payloadFactory>
<send/>