I found this address on the instructions for Oklahoma Form 538-S:
Filing instructions and due date:
If you are required to file an Oklahoma Income Tax Return, claim the sales tax refund as a credit on your tax return, Form 511, and enclose this signed form. Your return claiming the sales tax credit must be filed no later than April 18th. (See note at bottom of page).
If you are not required to file an Oklahoma Income Tax Return, this form must be filed no later than June 30. If you have withholding or made estimated payments and are filing for a refund on Form 511, you must claim the sales tax credit on your return and enclose this signed form. If you are not filing an income tax return, mail this completed and signed form to: Oklahoma Tax Commission, Post Office Box 26800, Oklahoma City, OK 73126-0800.
https://www.ok.gov/tax/documents/538-S-15.pdf
If this does not fully answer your question, or this form is for a state other than Oklahoma, kindly post additional details below.