Thanks for your information. I'm in struggling that the finished message showed blank. It seems the finished page still rendered with my UserDefinedForm.ss instead of ReceivedFormSubmission.ss. One of my trial is to add "render" at the end of finished() like below:
'Content' => $this->customise(array(
'Submission' => $submission,
'Link' => $referrer
'Form' => '',
it can make the page rendered with ReceivedFormSubmission.ss but still shows nothing, I'm thinking the value of $OnCompleteMessage in ReceivedFormSubmission.ss is correctly got.
Would be thankful if any suggestion provided.