Use task-last-id to get the ID for the last provisioning task.

Request 

<packet>
  <extension>
    <call>
      <plesk-multi-server>
        <task-last-id/>
      </plesk-multi-server>
    </call>
  </extension>
</packet>

Response 

<packet>
  <extension>
    <call>
      <result>
        <status>ok</status>
        <plesk-multi-server>
          <task-last-id>
            <id>123</id>
          </task-last-id>
        </plesk-multi-server>
      </result>
    </call>
  </extension>
</packet>