This is an example of using CancellationTokenSource with Fabulous--the Xamarin.Forms Elmish framework.

FabTestCTS.fs

namespace FabTestCTS

open System.Diagnostics
open Fabulous.Core
open Fabulous.DynamicViews
open Xamarin.Forms

module Cmd =
    let ofAsyncMsgOption (p: Async<'msg option>) : Cmd<'msg> =
        [ fun dispatch -> async { let! msg = p in match msg with None -> () | Some msg -> dispatch msg } |> Async.StartImmediate ]

module App = 
    type Model =
        { Test : string }

    type Msg = 
        | Start
        | Cancel

    let initModel =
        { Test = "test" }

    let init () =
        initModel, Cmd.none

    let mutable cts = new System.Threading.CancellationTokenSource()

    let work =
        async {
            while true do
                printfn "Working..."
                do! Async.Sleep 1000
        }

    let start = async {
        cts.Cancel()
        cts <- new System.Threading.CancellationTokenSource()
        printfn "Start"
        Async.StartImmediate(work, cts.Token)

        return None
    }

    let cancel = async {
        printfn "Cancel"
        cts.Cancel()

        return None
    }

    let update msg model =
        match msg with
        | Start -> model, Cmd.ofAsyncMsgOption start
        | Cancel -> model, Cmd.ofAsyncMsgOption cancel

    let view (model: Model) dispatch =
        View.ContentPage(
          content = View.StackLayout(padding = 20.0, verticalOptions = LayoutOptions.Center,
            children = [ 
                View.Button(text = "Start", command = (fun () -> dispatch Start), horizontalOptions = LayoutOptions.Center)
                View.Button(text = "Cancel", command = (fun () -> dispatch Cancel), horizontalOptions = LayoutOptions.Center)
            ]))

    // Note, this declaration is needed if you enable LiveUpdate
    let program = Program.mkProgram init update view

type App () as app = 
    inherit Application ()

    let runner = 
        App.program
#if DEBUG
        |> Program.withConsoleTrace
#endif
        |> Program.runWithDynamicView app

#if DEBUG
    // Uncomment this line to enable live update in debug mode. 
    // See https://fsprojects.github.io/Fabulous/tools.html for further  instructions.
    //
    //do runner.EnableLiveUpdate()
#endif    

    // Uncomment this code to save the application state to app.Properties using Newtonsoft.Json
    // See https://fsprojects.github.io/Fabulous/models.html for further  instructions.
#if APPSAVE
    let modelId = "model"
    override __.OnSleep() = 

        let json = Newtonsoft.Json.JsonConvert.SerializeObject(runner.CurrentModel)
        Console.WriteLine("OnSleep: saving model into app.Properties, json = {0}", json)

        app.Properties.[modelId] <- json

    override __.OnResume() = 
        Console.WriteLine "OnResume: checking for model in app.Properties"
        try 
            match app.Properties.TryGetValue modelId with
            | true, (:? string as json) -> 

                Console.WriteLine("OnResume: restoring model from app.Properties, json = {0}", json)
                let model = Newtonsoft.Json.JsonConvert.DeserializeObject<App.Model>(json)

                Console.WriteLine("OnResume: restoring model from app.Properties, model = {0}", (sprintf "%0A" model))
                runner.SetCurrentModel (model, Cmd.none)

            | _ -> ()
        with ex -> 
            App.program.onError("Error while restoring model found in app.Properties", ex)

    override this.OnStart() = 
        Console.WriteLine "OnStart: using same logic as OnResume()"
        this.OnResume()
#endif