Find all specific types, with reflection, in assemblies loaded by the application

I have a system where there are commands, for a chat bot, scattered in different part of the code. I find, and register, them all through reflection using this code:

member private this.RegisterCommands() =
    let t = typeof<IBotCommands>
    t.Assembly.GetTypes()
    |> Seq.filter (fun x -> x.GetInterface(t.Name) <> null)
    |> Seq.iter (fun x ->
        (
            let i = (Activator.CreateInstance(x)) :?> IBotCommands
            botCommands.[i.Name()] <- i
            i.Initialize(this)
        )
    )

It's in F#, but I think the method calls give away what it does if you're more familiar with C#.

So, I do Assembly.GetTypes, keep the ones where GetInterface has the interface name I'm looking for and then I instantiate them.

This works well, but only within its own assembly. My question is how can I expand this to all assemblies loaded by the app (but the system ones to not waste time)?