There is no way currently to use a connection string with the async driver to connect to a cluster via SSL. This is because the default async stream type, AsynchronousSocketChannelStream, does not support SSL. But in order to use NettySteam, you need to configure the async MongoClient with a NettyStreamFactoryFactory, which can not be done via connection string.

To make this simple use case easier, add connection string support for streamType=netty for the asynchronous driver.