<p:commandButton value="Delete" update="testPlanetree" id="deleteBtn"
   disabled="#{projectTestManagementMB.disable}" oncomplete="deleteConfirmation.show()"
    action="#{projectTestManagementMB.testFn}"/>


 <p:confirmDialog id="confirmDialog" message="#
  {projectTestManagementMB.deleteConfirmationMsg}"
    header="Confirming Deleting Process" severity="alert"
   widgetVar="deleteConfirmation">

    <p:commandButton id="confirm" value="Yes Sure" update="messages"
     oncomplete="deleteConfirmation.hide()"    />

      <p:commandButton id="decline" value="Not Yet"
       onclick="deleteConfirmation.hide()" type="button" />

     </p:confirmDialog>

private String deleteConfirmationMsg;//with getters and setters
    public void testFn(){
       deleteConfirmationMsg="do you want to delete ...";
    }

<p:commandButton ... update="confirmDialog testPlanetree">

<p:commandButton value="Delete" update="testPlanetree" id="deleteBtn" actionListener="#
         {projectTestManagementMB.testFn}"
       disabled="# {projectTestManagementMB.disable}"

       oncomplete="deleteConfirmation.show()"  />