Code for screencast at https://www.youtube.com/watch?v=V-fNIOLYv88

GitHub.kt

package controller

import models.User
import tornadofx.Controller
import tornadofx.Rest

class GitHub : Controller() {
    val api : Rest by inject()

    init {
        api.baseURI = "https://api.github.com"
    }

    fun login(user: User): Boolean {
        api.setBasicAuth(user.usernameProperty.value, user.passwordProperty.value)
        return api.get("user").consume().ok()
    }

}

LoginScreen.kt

package view

import app.Styles.Companion.login
import controller.GitHub
import javafx.geometry.Orientation.VERTICAL
import javafx.scene.control.Alert.AlertType.WARNING
import javafx.scene.control.Button
import javafx.scene.control.ProgressIndicator
import models.User
import models.UserModel
import tornadofx.*

class LoginScreen : View() {
    override val root = Form().addClass(login)
    val model = UserModel(User())
    val github: GitHub by inject()

    init {
        title = "GitHub Login"

        with(root) {
            fieldset {
                labelPosition = VERTICAL

                field("Username") {
                    textfield(model.username).required(message = "Input your user name")
                }
                field("Password") {
                    passwordfield(model.password).required()
                }
            }

            button("Log in") {
                setOnAction {
                    login()
                }
            }
        }
    }

    private fun Button.login() {
        if (model.commit()) {
            graphic = ProgressIndicator()
            runAsync {
                github.login(model.user)
            } ui { success ->
                graphic = null
                if (success) replaceWith(ProtectedScreen::class, ViewTransition.SlideIn)
                else alert(WARNING, "Login failed", "Check your credentials")
            }
        }
    }
}

MyApp.kt

package app

import app.Styles
import view.LoginScreen
import tornadofx.App
import tornadofx.importStylesheet

class MyApp: App() {
    override val primaryView = LoginScreen::class

    init {
        importStylesheet(Styles::class)
    }
}

ProtectedScreen.kt

package view

import javafx.scene.control.Label
import javafx.scene.layout.StackPane
import tornadofx.View

class ProtectedScreen : View() {
    override val root = StackPane(Label("You are logged in"))

    init {
        title = "Protected Screen"
    }
}

Styles.kt

package app

import javafx.scene.text.FontWeight
import tornadofx.Stylesheet
import tornadofx.box
import tornadofx.cssclass
import tornadofx.px

class Styles : Stylesheet() {
    companion object {
        val login by cssclass()
        val loginWidth = 300.px
    }

    init {
        form and login {
            padding = box(25.px)
            fontSize = 20.px
            fontWeight = FontWeight.BOLD
            prefWidth = loginWidth
            button { prefWidth = loginWidth }
            progressIndicator {
                prefWidth = 16.px
                prefHeight = prefWidth
            }
        }
    }
}

User.kt

package models

import javafx.beans.property.SimpleStringProperty
import tornadofx.ViewModel

class User {
    val usernameProperty = SimpleStringProperty()
    val passwordProperty = SimpleStringProperty()
}

class UserModel(var user: User) : ViewModel() {
    val username = bind { user.usernameProperty }
    val password = bind { user.passwordProperty }
}