APHIS (Agricultural Pant Health Inspection Service) is the US agency that oversees the ISPM program. I know, we al thought an aphis was an insect a few years ago.
APHIS, in turn, delegates the implementation to the ALSC (American Lumber Standards Committee).
Grading agencies, such as WCLIB or WCLIB, get their authority to issue grade stamps from the ALSC.
Therefore, your grading agency is the go to organization. They will issue the authority for you to use the HT stamp.