Unfortunately, NostrSharp is not a pre-existing library or framework. It's just a conventionally come fullname for the example given combining name of the platform 'Nostr' and the programming language 'C#'.

That being said, you can still use Nuget to manage packages and dependiences in C# projects. Here's how to download/install Nostr via Nuget Package manager console;

1. Open up your project solution

2. Click on Tools -> Library Package Manager -> Package Manager Console

Now in NuGet console type,

```

Install-Package NBitcoin

```

After installation of one among recommended bitcoin wallet libraries (NBitcoin is one), proceed with fetching it from gateway following guidelines