First, you need to login to EC-Ship and then associate your permit account by supplying the Permit No and its activation code in "My Permit" page. After the association, you are able to select the permit no. when preparing your bulk posting statement.
The activation code is provided by Hongkong Post and has been mailed to you before. If you didn't receive or lost the code, please contact hkpo@hkpo.gov.hk.