process.xml

<property as="xs:string" name="oxf.fr.detail.process.send.*.*">
    require-uploads
    then validate-all
    then send(uri = "http://validate")
    then success-message(message = "Everything good")
    recover error-message(message = "Data invalid")
</property>

test-service.xhtml

<xh:html xmlns:xh="http://www.w3.org/1999/xhtml"
      xmlns:xf="http://www.w3.org/2002/xforms"
      xmlns:xxf="http://orbeon.org/oxf/xml/xforms"
      xmlns:ev="http://www.w3.org/2001/xml-events"
      xmlns:xs="http://www.w3.org/2001/XMLSchema"
      xmlns:fr="http://orbeon.org/oxf/xml/form-runner">
    <xh:head>
        <xf:model>
            <xf:instance><gaga/></xf:instance>
            <xf:submission id="respond" method="post" resource="/fr/service/custom/orbeon/echo" replace="all"/>
            <xf:action ev:event="xforms-model-construct-done">
                <!--<xf:action type="xpath">frf:sendError(500)</xf:action>-->
                <xf:send submission="respond"/>
            </xf:action>
        </xf:model>
    </xh:head>
</xh:html>