Gradle multi project Plugin with id 'jettyEclipse' not found

I want to add the jettyEclipse plugin to a multi project in Gradle, according to the documentation, we should add the following snippet to the build script

    buildscript {
        repositories {
        dependencies {
            classpath (group: 'com.sahlbach.gradle', name: 'gradle-jetty-eclipse-plugin', version: '1.9.+')
    apply plugin: 'jettyEclipse'

This works in a single Gradle project, but failed when do this in a multi project. I add the snippet to the subprojects {} block but get the following error.

The reason is the buildscript is for the build.gradle file itself, the solution is put it in the build.gradle file of the sub project.