For C# gRPC you can avoid code-gen and use existing GetAccessToken
DTO with the GrpcServiceClient code-first generic Service Client.
Which wont be possible when using a grpc code generated proxy, i.e. in a different language like Dart.
But this looks like a use-case for the new ForceInclude Metadata Types v6 feature:
public override void Configure(Container container)
{
Metadata.ForceInclude = new() {
typeof(GetAccessToken),
};
}