How can I get window.location.href in Elm?

There is elm-history package with the location function for this, but it's deprecated and doesn't exist for 0.18 version.

Then you might want to use elm-navigation package and explicitly store the current location in your model.

Please have a look at this example. A program with navigation can be created via:

Navigation.program UrlChange
    { init = init
    , view = view
    , update = update
    , subscriptions = (\_ -> Sub.none)

UrlChange here is a type of message, which triggers on every url change, so you can process it and set the current location:

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        UrlChange location ->
            ( { model | location = location }
            , Cmd.none

And then purely get the location.href wherever the model is accessible.

In the provided application, this place is view: viewLocation model.location

In your application, it's, for example, something like this:

url model =
    model.location.href ++ "/api/tasks"

Use URL Builder to link to anoter page on your site

There's no need to specify the base url:

import Url.Builder exposing (absolute)
url = absolute [ "api", "tasks" ] []
-- results in "http://localhost:8080/api/tasks"
-- if working in your developer environment
-- the URL will automatically change in production
-- to the correct URL assuming you don't have any crazy set ups

And you definitely will not need to worry if your server URL changes. So you can easily switch from a development to production/staging environments without any additional configuration.

Whilst the above answers your question, I think there is a more straightforward solution to the problem:

If the application code is being served from the same server (URL) as the API you want to access you don't need to specify the server - just the root relative path for your api i.e. you can make requests to /api/tasks from your elm code and the browser will sort out the rest for you.

This is how I addressed the problem in my deployed code.