How to add AdMob to Android app

buildscript {
    repositories {
        google()
        jcenter()

    }
    dependencies {
        classpath 'com.google.gms:google-services:4.3.2'
    }
}

class Version {
    class Firebase {
        static def analytics = "17.2.0"
        static def ads = "18.2.0"
    }
}

dependencies {
    implementation "com.google.firebase:firebase-analytics:$Version.Firebase.analytics"
    implementation "com.google.firebase:firebase-ads:$Version.Firebase.ads"
}

apply plugin: 'com.google.gms.google-services'

<manifest>
    <application>
        <!-- Sample AdMob App ID: ca-app-pub-3940256099942544~3347511713 -->
        <meta-data
            android:name="com.google.android.gms.ads.APPLICATION_ID"
            android:value="[ADMOB_APP_ID]"/>
    </application>
</manifest>

class MyApplication: Application() {
    override fun onCreate() {
        super.onCreate()

        MobileAds.initialize(this)
    }
}

<com.google.android.gms.ads.AdView
        xmlns:ads="http://schemas.android.com/apk/res-auto"
        android:id="@+id/adView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        ads:adSize="BANNER"
        ads:adUnitId="ca-app-pub-123456/123456"
        ads:layout_constraintBottom_toBottomOf="parent"
        ads:layout_constraintLeft_toLeftOf="parent"
        ads:layout_constraintRight_toRightOf="parent"/>

import com.google.android.gms.ads.AdRequest
import com.google.android.gms.ads.AdView

val request = AdRequest.Builder().build()
adView.loadAd(request)

dependencies {
    implementation 'com.google.android.gms:play-services-ads:18.2.0'
}

android {
    defaultConfig {
        multiDexEnabled true
    }
}

dependencies {
    implementation 'com.android.support:multidex:1.0.3'
}