Hello Roger Roger,
Thank you for reaching out to Microsoft Support!
You can connect using the client secret as follows:
$ClientSecretCredential = Get-Credential -Username "Client_Id"
# Enter client_secret in the password prompt.
Connect-MgGraph -TenantId "Tenant_Id" -ClientSecretCredential $ClientSecretCredential
Reference document:
https://github.com/microsoftgraph/msgraph-sdk-powershell/blob/dev/docs/authentication.md
Hope this helps.
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