With the Kotlin script, we should add the same dependencies. Your build.gradle.kts file will look as follows:

import org.gradle.api.plugins.ExtensionAware

import org.junit.platform.gradle.plugin.FiltersExtension
import org.junit.platform.gradle.plugin.EnginesExtension
import org.junit.platform.gradle.plugin.JUnitPlatformExtension

buildscript {
    dependencies {
        classpath("org.junit.platform:junit-platform-gradle-plugin:1.0.0")
    }
}

apply {
    plugin("org.junit.platform.gradle.plugin")
}

configure {
    filters {
        engines {
            include("spek")
        }
    }
}

dependencies {
    testCompile("org.jetbrains.spek:spek-api:1.1.5")
    testRuntime("org.jetbrains.spek:spek-junit-platform-engine:1.1.5")
}

fun JUnitPlatformExtension.filters(setup: FiltersExtension.() -> Unit) {
    when (this) {
        is ExtensionAware -> extensions.getByType(FiltersExtension::class.java).setup()
        else -> throw Exception("${this::class} must be an instance of ExtensionAware")
    }
}
fun FiltersExtension.engines(setup: EnginesExtension.() -> Unit) {
    when (this) {
        is ExtensionAware -> extensions.getByType(EnginesExtension::class.java).setup()
        else -> throw Exception("${this::class} must be an instance of ExtensionAware")
    }
}