Power BI embedded with SSAS directe connection

Hi dear,
Could you please be more clear. I am not a .NET developer, I just want to use the sample on th GitHub.
My report work fine when uploaded to Power Bi Service.

I guess that the Patch to update the datasource is already applied on the Sample code :
 

// Update the datasource with the specified credentials
await client.Gateways.PatchDatasourceAsync(workspaceCollectionName, workspaceId, datasources.Value[datasources.Value.Count - 1].GatewayId, datasources.Value[datasources.Value.Count - 1].Id, delta);

My concerne is with the parameter I should use:

// Reset your connection credentials
                var delta = new GatewayDatasource
                {
                    CredentialType = "Basic",
                    BasicCredentials = new BasicCredentials
                    {
                        Username = username,
                        Password = password
                    }
                };

I guess that user name and password are those used for my datasource on the Gateway.
The VS debugger shows a null value for datasources variable.
Should I enter a connection string ?