.net maui, how do I attach a client certificate to a SOAP request for mutual TLS?

Question

I have .NET MAUI app (VS2022 17.3.4 SDK 6.0.401) targeting Windows and Android as a learning project. It has one button on the UI, when clicked it calls a SOAP 1.2 webservice that uses mutual TLS requiring a client certificate.

In windows it works OK. On Android, I don't know how to fetch the client certificate from the store. My SOAP client (derived from ClientBase) wants a client certificate in ClientCredentials. Working on Windows is:

client.ClientCredentials.ClientCertificate.SetCertificate(
        StoreLocation.CurrentUser,
        StoreName.My,
        X509FindType.FindBySerialNumber,
        "3d577021");

When it runs on Android the above line throws an exception:

System.InvalidOperationException: 'Cannot find the X.509 certificate using the following search criteria: 
  StoreName 'My', StoreLocation 'CurrentUser', FindType 'FindBySerialNumber', FindValue '3d577021'.

I think because the store is not "My"? Or maybe I need to configure the app in some way to access the certificate store.
I think the Client certificate is installed correctly. I used Android Settings, Encryption and Credentials -> Install from SD card. This is new to me so maybe that's wrong.

I would appreciate any pointers to a solution or documentation.