There are a few ways that you can get that bounce card look in your renders. You can easily use an IBL or reflected environment that uses and HDR. The problem with that is that it will be really difficult to aim the thing to get the reflections exactly where you want them.

how you can create a reflection on the screen of a device, like smartphone, tablet or tv screen

Of course you can simply create a reflective bounce card out of a polygon plane, but while aiming the plane, you might find that you will be able to see where the card ends in the animation, depending on the angles.

The solution is pretty simple, as shown to use by CyBear. Use a circular shape for your bounce cards. This will give you the ability to aim it really easily, just like the polygon plane, but you will never see the bounce card reflections end top-to-bottom.

Adding a gradient to the card will give you more interest to the reflections in the scene, and is a great way to quickly place highlights and reflections exactly where you want them.