At present, the stylus eraser button should be auto-assigned, but only on some devices and versions of Joplin. In particular,
- On Android, it should work with Joplin >= 3.0.4 (pre-release), but, because the image editor runs in a Chromium-based WebView, it might be broken on Samsung hardware. Also see this GitHub issue.
- On iOS: Untested, should work with Joplin >= 13.0.2 (pre-release).