Launch app from a designated channel number

To directly answer your question, you cannot program an unused channel to do that. 

U can also use the voice search button to launch apps. 

@Ludwick577, thanks for your response. I've got a DirecTV Stream Box set up for some older family members who find new technology a little daunting - Netflix being on a channel number was a revelation for them, and they have been using it so much more than when we had a firestick set up for them (i.e. they never used it unless we were here to help). I can try to teach them the voice search, however I am doubtful that they will get on well with this.

Wow they must be really bad with technology. All they have to do is hit the voice remote button and say "netflix, hulu, etc" The voice button even has a microphone on it. Probably easier for them vs remembering a channel number. 

Thanks @litzdog911.