Rest API :: Search by client Variables

Hello,

How can I build a SearchQuery that filters Client Varibles?

Consider this:

<ProcessInstance>
      <processUUID>
         <value>XXXXXXXX--1.1</value>
      </processUUID>
      <instanceUUID>
         <value>XXXXXXXX--1.1--278691</value>
      </instanceUUID>
      <rootInstanceUUID>
         <value>XXXXXXXX--1.1--278691</value>
      </rootInstanceUUID>
      <nb>278691</nb>
      <lastUpdate>1442324995913</lastUpdate>
      <state>STARTED</state>
      <endedDate>0</endedDate>
      <startedBy>admin</startedBy>
      <startedDate>1441187449615</startedDate>
      <isArchived>false</isArchived>
      <**clientVariables**>
         <entry>
            <string>MY_CUSTOM_FIELD</string>
            <string>1507280132</string>
         </entry>
        .....

I would like to build a query, to be used on Bonita's REST API:

=> bonita-server-rest/API/queryRuntimeAPI/searchByMaxResult

<SearchQueryBuilder>
    <index class="org.ow2.bonita.search.index.ProcessInstanceIndex"/>
    <query>
        <org.ow2.bonita.search.Criterion>
            <builder reference="../../.."/>
            <fieldName>processDefUUID</fieldName>
            <value>XXXXXXXX*</value>
        </org.ow2.bonita.search.Criterion>
        <string> AND </string>
        <org.ow2.bonita.search.Criterion>
            <builder reference="../../.."/>
            <fieldName>MY_CUSTOM_FIELD</fieldName>
            <value>1507280132</value>
        </org.ow2.bonita.search.Criterion>
    </query>
</SearchQueryBuilder>

How can I build a criterion element that filters that Client Variable named MY_CUSTOM_FIELD?

Thank you!

I'm using Bonita v5.9.1