It’s not available in the app stores. It’s more of private browser on your phone. If you open up your browser menu when on the forum you should see an ‘install app’ or ‘add to home screen’ option. You can also clear your cookies, and that should make the banner asking if you want to install it show up again. 
@Blonde_Bunny likes to use the Discourse Hub app, as that’s a little more discreet: