I'm developping a Windows Store 8 Application. For this I would like some custom TextBox controls. How can I make something like in the picture? (XAML) specifically those corners

I'm developping a Windows Store 8 Application. For this I would like some custom TextBox controls. How can I make something like in the picture? (XAML) specifically those corners

Just make yourself a custom Control Template (Yes the link says Silverlight, but you wont see much difference in the win8 XAML etc) and you could use a Path for your backgrounds and even add things like color changing for the different MouseOver, Focused, States etc.
<Path Data="M130.32379,0 L142.74992,0 L157.743,15.01704 L142.73584,30.000004 L130.30968,30.000004 L0.31890577,29.99987 L15.159052,15.183544 L0,0.00013974622 z"
Fill="#FF1FCD99" Height="30" Width="150" Stretch="Fill" UseLayoutRounding="False"/>
Once you have your custom Control Template for your TextBox TargetType it's just a matter of applying it where needed with a Style tag pointing to your resource like Style="{StaticResource BlahTextBox}".
Hope this helps.