Hi mkatoma,
Welcome to the Roku Community.
You can of course cast to your Roku TV what you are already watching on your mobile device. To use casting, the supported app must be installed on both your mobile device and your Roku streaming device. You must also make sure to connect your phone or tablet to the same wireless network as your Roku device.
You can do this wirelessly with supported apps like YouTube and Netflix by opening the casting icon on a mobile device on your phone or tablet and selecting your Roku device. The channel will launch automatically on your Roku device and playback will begin.
Please read the detailed article about: How to cast apps like YouTube or Netflix from your phone to a TV
Keep us posted on how it goes.
Regards,
Nimfa