You can install this specific version from NuGet.org or GitHub Packages using either of these commands:
Using the Package Manager Console:
PM> Install-Package MikaelDui.Net.WebSockets.Wamp --Version 0.0.12
Using the .NET CLI:
dotnet add package MikaelDui.Net.WebSockets.Wamp --version 0.0.12
But it's preferable to use floating versions, see the Microsoft Docs.