If you want to set up in-app advertising with an AdMob account, you will need to insert your AdMob key.
To find your AdMob Key, log into your AdMob account and click on the Monetize tab on the top. Then click the Monetize new app button:
Click on Add your app manually and select the platform (Android or iOS):
Click on the Banner button at step 2:
Add the Unit name:
And finally collect your AdMob key as shown in the red highlighted box (barred part):