Need to double-check but currently my guess of what we need to do is:

• Our system must list mms-api-meshonly as its consumer.
• Have the gRPC client extend AbstractGrpcClient with DualAuthHeaderCallCredentials (apparently important)
•  Add client. {service-name}.grpc.address per environment to the monolith conf files (like server/conf/conf-local.properties for example)

• Bind the client as @Singleton, inject and call from the Admin API endpoint (like the MeterAdminClient is doing)

There might be a few more things to configure.
Check the existing code base and reach out to our CNCP champion as you start working on this ticket.