The faq tells you how to do it with a virtual device. Not a real button. 
Once you have created the virtual device, you can then use it in both a smartthings routine And an Alexa routine.
I’m pretty tired right now and we just went over all this in detail in a different poster’s thread earlier today, so if it’s OK, I’m just going to link to the FAQ again. There are two different methods, one for if you have a smartthings/Aeotec hub, and one for if you don’t.
Take a look at this faq and see if it helps. If not, post, any questions, and someone will be able to help you.
FAQ: Can I trigger an Echo Action without Speaking to It?
Note that this method is just a way of initiating an Alexa action from smartthings. But as long as an Alexa voice command can get you to the TV program you want, you can include that in a smartthings automation using the FAQ method.