How to set our app's master page as the default for every new topic

I've created a custom master page for our app's topics. The only change from factory default is to add Breadcrumbs. It works and I can manually assign it as each new topic's master page. But it would save a lot of time and be more fail-safe if the app's master page could be assigned as the default for every new topic. If that is possible, how to set it up? I've drilled into all of the master page config panels and nothing jumps out. Thanks! Nick