The drivers written by MongoDB don't understand Shared Throughput.
If you want a .NET library that can create a Mongo DB database with shared throughput and collections underneath, an option you can use is the Azure Management Libraries for .NET. There is a repo with a bunch of samples that show how to create and manage Cosmos DB resources directly against its control plane, cosmos-management-net
Here is a specific sample for creating MongoDB resources in Cosmos DB MongoDB.cs.
I would use the generated libraries. The fluent ones are not as well supported.