How do I use the "onunload" event to send a message to the server in SAFE Stack?

Question

I've got an application built using the SAFE stack where active clients have corresponding objects on the Server side. When a user closes their browser window, I would like a message to be sent to the server to delete that object.

I'm trying to use the "onunload" event like this:

module WindowEvents =
    let unloadSub _ =
        let setUnloadEvent dispatch = 
            Browser.Dom.window.onunload <- (fun _ -> dispatch CloseEvent)
        Cmd.ofSub setUnloadEvent

and I add that to the subscriptions:

Program.mkProgram Model.init Model.update View.render
|> Program.withSubscription WindowEvents.unloadSub

The message is handled in the usual Elmish way and is forwarded on to the Server via the api. However, I find that the message only rarely arrives server-side. Is there something I can do to make this more reliable? Or is there an alternative approach that would be better?