REST API - Authentication/Logon - Missing Parameter(s)

I am trying to connect to a OneStream Cloud environment for which the REST API has been enabled and tested by OneStream Support.

I am working in PostMan and following the REST API guide at System Guides > REST API Implementation > REST API Overview > Authentication, but i am hitting an error "One or more required parameters are missing."

I can retrieve the AzureAD token fine, but when I then try and use it to authenticate with OneStream (expecting the SI to use later on), I get the above error message returned (along with a HTTP/400 Bad Request).

As far as I can see I am not missing any Parameters as per that section of the REST API Guide.

The POST request is to:

https://xxxx.onestreamcloud.com/onestreamapi/api/authentication/logon?api-version=7.2.0

Params: api-version=7.20

Authorization: Bearer Token returned from Azure AD (via a PostMan variable)

Content-Type: application/json

Body: BaseWebServerURL: https://xxxx.onestreamcloud.com/onestreamweb

What am I missing?

Thanks

Steve