The F-droid flavor of the app uses UnifiedPush, the protocol behind ntfy. With Ntfy installed, just go to notifications or the app settings and activate it. That should be all you need to make it work. 

Keep in mind that the free version of Ntfy is limited to just a 100 notifications and there are a lot of rate limiting on the free version. This is particularly bad if Ntfy is also serving notifications for other apps.