Credentials are not stored in the model, they are set in the PBI Service.

See

https://github.com/microsoft/PowerBI-Developer-Samples/blob/master/PowerShell%20Scripts/Patch-SQL-Datasource-Credentials-and-Refresh.ps1

https://www.data4v.com/automated-updation-of-power-bi-data-source-credentials/

https://learn.microsoft.com/en-us/power-bi/developer/embedded/configure-credentials?tabs=sdk3