This tutorial explains how to build a command-line client for FusionAuth using C# and the .NET Core libraries. The client is designed to create users in FusionAuth, with options to associate them with specific applications. The tutorial covers setting up FusionAuth, creating a user request object, and registering a user with an application. It also discusses the use of arbitrary key-value pairs in the data field and the registration process. The API choice used in this tutorial is the "Create a User" API, which works well for creating one user at a time. However, for bulk imports or migrations, other APIs like Bulk Import and Migrate Users should be considered. The client library provides a convenient way to interact with FusionAuth's APIs, making it easier to extend its functionality.