Android studio 2.3.3 cannot write to external storage of emulator

I am an absolute beginner with Android. My app writes to external storage. When I execute my program on a real device, it executes perfectly. So no problem with the code.(manifest file also ok). However, when I test with the emulator, my program cannot write to the external storage of the emulator. I read that I have to give permission to my app in order to write to the external storage of the emulator. I have looked in great detail but I do not find how to do this on the emulator of Android Studio 2.3.3. Thank you for helping this absolute beginner.