Struggling to retrieve user choice from a dropdown using SAFE

I am sorry if this is the wrong forum to post this question - if so, I would be very grateful if anyone could point me to a more appropriate place.

I have finally taken the plunge to try and build a website using SAFE. It has been slow, but I am persevering as I love F Sharp and I am looking forward to being able to write the back-end code in F#. But there is so much that is new to me in full stack development. I am first trying to set up the front-end to accept some simple user input. Using code that is equivalent to the code below, I can update the state (and send to the back-end) user input that is received via radio buttons and also text taken from a textarea element. I cannot, though, extract user input entered via a element.

The code snippets compiled below, I hope, are all the relevant sections. The program has no flagged errors, but the output is not as desired. I was hoping that the indx parameter in the ‘update’ section would pick up the index of the newly selected item in the element when that element is changed, but it does not seem to pick up anything. I have tried altering the line prop.onChange (plainValuesChanged >> dispatch), but no luck so far.
I am slightly nervous about asking where I am going wrong because I am so conscious of how much I don’t know, but I am hoping that there is an answer out there that will get me over this hurdle …

let plainValues = [|
    0, "plain1"
    1, "plain2"
    2, "plain3" |]

let update (msg: Msg) (model: PageModel) : PageModel * Cmd<Msg> =
    match model, msg with
    | _, plainValuesChanged indx ->   { model with
                                            serverState = Idle
                                            plainValue = snd plainValues[indx]) 
                                            }, Cmd.none

let view (model: PageModel) dispatch =
    Html.div [
        Bulma.control.div [
             prop.children [
                        prop.onChange (plainValuesChanged >> dispatch) 
                        prop.children [
                                                        for (value, text) in plainValues do
                                                            Html.option [
                                                                prop.value value
                                                                prop.text text ] ] ] ] ] ]