### What did you expect to happen?
To send it correctly
### Relevant log output
```Shell
Please include a case-sensitive header of Authorization: Basic <YOUR-REST-API-KEY-HERE> or Bearer token="<YOUR-REST-API-KEY-HERE>" with a valid REST API key.'], 'reference': ['https://documentation.onesignal.com/docs/accounts-and-keys#section-keys-ids
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[X] I agree to follow this project's Code of Conduct
What happened?
the param include_external_user_ids requires the REST API KEY. I have checked the documentation and I don't see how can I add it to the client.
I tried using this:
api_client = onesignal.ApiClient(config, "Authorization", f"Basic {api_key}")
But it does not send the headers to the API.
Anything am I missing?
Steps to reproduce?
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