Using the Rectangle Tool (M), create a 250 x 160px shape, fill it with the linear gradient shown below and go to Effect > Stylize > Rounded Corners. Enter a 10px radius, click OK and go to Object > Expand Appearance. Move to the Layers panel (Window > Layers), double-click on the existing shape and simply name it "main". Select "main" and make a copy in front (Control + C > Control + F), you'll need it later.