Unable update Datasource Credentials using credential type as OAuth2 using Power BI Rest API

Hello,

While trying to update a Datasource credentials for oAuth2 authentication using the Power BI Rest API, it throws an error saying the credentials are invalid.

Error: The credentials provided for the Web source are invalid. 

(https://docs.microsoft.com/en-us/rest/api/power-bi/gateways/updatedatasource#oauth2-credentials-example ) 

As shown in this documentation, it requires an access token to be passed while using the API to change the data source credentials. It is not clear as to what type of access token needs to be used for this and how to generate the same.

Any help would be appreciated plus official documentation needs to be updated to include more details related to generation of access token.