Instead of a physical key, you can display a button on the screen and take a screenshot with that button.

Provides the following additional functions.
1. Cut the top bar and bottom bar
2. Hide the top bar (date/time, user text)
3. Optimizing Insertion
4. Resize

---
App icon license
Source: https://iconarchive.com/show/android-lollipop-icons-by-dtafalonso/Camera-icon.html
Artist: dtafalonso
License: CC Attribution-No Derivative 4.0
Commercial usage: Allowed