Unable to update user-password using API UpdateDatasourceWithHttpMessagesAsync

We moved the dashboards from v1 (workspace collections) to v2 (app workspaces). In doing so we have a deployment process that called the c# powerbi API to update the datasource username/passwords. In V1 there was a PatchDatasource method. In V2 this is non-existant. In V2 there is a UpdateDatasourceWithHttpMessagesAsync which I have tried to use but unsuccessful.

I have so far found no actual examples of how to call this successfully. I have found several forum posts but none if this is fruitful. 

The closest example was found at https://community.powerbi.com/t5/Developer/How-do-I-update-the-Credentials-after-I-upload-a-report-to-POWER/td-p/223243

This example provides a separate method to encrypt basic credential but gives no information on how to find the gateway public key or gateway public key modulus. In trying to query for the gateway using the c# api method GetGatewaysWithHttpMessagesAsync this always returns zero gateways. So cannot go any futher using this partial example.

The other url's which I have looked at where other users are wanting to find the gateway public keys or having issues with updating the datasource credentials are below. And I see the same similar issues I am addressing above.

https://community.powerbi.com/t5/Developer/Update-Credentials-C/m-p/271635

https://community.powerbi.com/t5/Developer/Automating-Power-BI-Gateway-administration-by-using-Powershell/m-p/366494#M10914

So the main question, is there a full codeexample of where this has been successfully done that can be reviewed? 

Thanks.