Theresa,
From the screenshot, it looks like the name of your attributes in the body does not match what we are expecting. You should be passing in
tin, not
taxpayerIdentificationNumber. We have examples of queries on our documentation here which you can use.
Furthermore, it appears that the TIN you are attempting to use is only 8 digits. A TIN is 9 digits in length. If you are using the OAuth token you use for the Submissions API, that is supported, but keep in mind you will need to be explicitly granted authorization for the resource you are trying to opt-in for.
Thanks,
Chris