Parse a single query param

ParseQueryParam.elm

import Url
import Url.Parser
import URL.Parser.Query

parseQuizNumber : String -> Maybe Int
parseQuizNumber urlString =
    let 
        dropPath : Url -> Url
        dropPath url =
            { url | path = "" }

        parseUrl : Url -> Maybe (Maybe Int)
        parseUrl = Url.Parser.parse (Url.Parser.query (Url.Parser.Query.int "quiz"))
        
    in
    case Url.fromString urlStr |> Maybe.andThen (dropPath >> parseUrl) of 
        Just (Just qid) ->
            -- The URL was valid, the parameter was present and contained an int:
            Just qid

        Just Nothing ->
            -- The URL was valid, we parsed it, but there was no "quiz" param, or it wasn't an int
            Nothing

        Nothing ->
            -- The URL was invalid
            Nothing
 

The next step to use is to modify your main so it looks something like this:

main : Program String Model Msg
main =
    Browser.document
        { init = \urlString -> ( init (parseQuizNumber urlString), Cmd.none )
        , subscriptions = subscriptions
        , update = update
        , view = view
        }

And, in index.html:

      var app = Elm.Main.init({
        node: document.querySelector('main'),
        flags: window.location.href
      });