Choose a specific app to open a particular file
You can only open files with apps that tell Android they support that kind of file. If the app tells Android it can open that kind of file, it'll show up in the list of possible apps.

If this app you mention can open this kind of file, the developer needs to change the app to tell Android it can do so, by adding an intent filter in its manifest. You should contact the developer and ask them to do so.