Well, I have answered my own question. It's quite easy. For anyone looking for this in the future:
<Grid x:Name="LayoutRoot" Background="Transparent">
<phone:WebBrowser x:Name="Browser"
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch"
Loaded="Browser_Loaded"
ScriptNotify="Browser_ScriptNotify"
NavigationFailed="Browser_NavigationFailed" />
<adduplex:AdControl x:Name="adDuplexAd"
AppId="your app id" Margin="163,0,157,400" IsTest="True"
/>
</Grid>