They used to call then Private Channels. Roku now calls them Non-certified Channels. That reflects the situation more accurately. Roku allows you to add these Non-certified (Private) Channels (apps) if the developer provides it. Roku won't provide them, because if they come from Roku, they're certified and accepted, and therefore not private apps.
So, the developer would need to provide that.
For what it's worth, I think what's going to end up happening is the developer will eventually want you to load the app on your phone, then mirror it on your Roku. Note that "mirroring" isn't the same thing as "casting." And I think you may have been provided casting instructions. I suspect those won't work, and it's mirroring you want for that app.
DBDukes
Roku Community Streaming Expert
Note: I am not a Roku employee.
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