To

build.gradle.kts

plugins {
    kotlin("jvm") version "1.3.61"
    id("application")
    id("org.openjfx.javafxplugin") version "0.0.8"
}

javafx {
    // Declare the javafx modules we need to use
    version = "13.0.2"
    modules = mutableListOf("javafx.controls", "javafx.graphics")
}

application {
    mainClassName = "your.full.path.filename"
}

group = "org.example"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
    maven { setUrl("https://oss.sonatype.org/content/repositories/snapshots") }
}

dependencies {
    implementation(kotlin("stdlib-jdk8"))
    implementation("no.tornado:tornadofx:2.0.0-SNAPSHOT")
}

tasks {
    compileKotlin {
        kotlinOptions.jvmTarget = "11"
    }
    compileTestKotlin {
        kotlinOptions.jvmTarget = "11"
    }
}