How do I add atomicfu to my kotlin-gradle file? (T...

# gradle

[Stephan Schroeder]

How do I add atomicfu to my kotlin-gradle file? (The documentation only mentions the groovy version: https://github.com/Kotlin/kotlinx.atomicfu#jvm)
I tried:

buildscript {
    repositories {
        mavenCentral()
    }
    ext {
        this["atomicfu_version"] = "0.14.4"
    }
    dependencies {
        classpath("org.jetbrains.kotlinx:atomicfu-gradle-plugin:${ext["atomicfu_version"]}")
    }
}

plugins {
    kotlin("jvm") version "1.4.0"
    application
    id("kotlinx-atomicfu")
}

dependencies {
    implementation("org.jetbrains.kotlinx:atomicfu:0.14.4")

The syntax checker is happy with this, but I get runtime expections:
• "Unresolved reference: ext"
• "Unresolved reference. None of the following candidates is applicable because of receiver type mismatch:
public inline operator fun kotlin.text.StringBuilder /* = java.lang.StringBuilder */.set(index: Int, value: Char): Unit defined in kotlin.text"
• "No set method providing array access"