picking a notification sound?

I'm using a new app on my iphone 6, a travel app called "Hopper". It sends me alerts at various times, and the sound that I hear is identical to my text sound, the one known as "chord". I'd like to give my Hopper app a different sound to distinguish it from incoming texts, but can see no way to do it.

I've looked at notifications, sounds, and the app itself. But see no way to assign a different sound to this App. Presumably one has the ability to do this, even on an iphone. But how?

 

Thanks for your reply. Would this mean the only way to distinguish it from my text sound would be to change the text sound, or would the app sound change along with it?