Ads on Windows Phone?

I would just like to clear something up, I hear two different ways to do ads on the Windows Phone.

1. Make a plugin and call the code from there.
2. Add the code like you would for a Windows Phone app into the xaml code AFTER the build creates the visual studio project.

Which one is better? Do both methods work property? Like could I get AdMob in the app if I added it in the XAML after the build?

Thank you.