NET452 (which is the oldest version that's still supported by microsoft) - there are NET40 forks you can use.
Make sure you install
reactiveui-winforms into your solution. If you don't,
you'll see very strange behavior like
OneWayBind not working correctly,
WhenAny observables not being automatically observed on the UI thread.
Gluck (https://github.com/gluck) has published an unoffical forms package which from a fork of ReactiveUI repo:
Note that reactiveui-*-events packages (neither the ones from Paul, nor Gluck's) have no adherence to ReactiveUI per-se, you can do RxUI without them, and you can use them without RxUI. They're simply "generated extension methods for every type exposing .Net events to expose corresponding IObservable wrappers", to save you the burden of writing the FromEvent/FromEventPattern yourself.
This winforms package contains helpers for every type in System.Windows.Forms.dll.