Hi @John K ,
Thank you for posting query in Microsoft Q&A platform.
If your Token value is going to be constant all the time, then you should consider storing it in Azure Key vault secret and use that Azure Key vault secret name as value in linked service.
If you are going to get token value dynamically and wants to pass that during runtime in source transformation, then create a parameter in data flow and use that parameter as value in Source transformation headers. Make sure to pass your token value into data flow parameter when calling Data flow using Data flow activity.
In your case, it says 401 error, that means the client request has not been completed because it lacks valid authentication credentials for the requested resource. So I believe either we are not passing valid token or we are not passing token in proper way. Kindly re-check above points in your case and let us know how it goes?
Also, kindly check using your token in any other API tool(such as postman) to make API and see how it behaves there?