Luckily as a developer enabling this is incredibly simple - it's one line in the AndroidManfiest.xml.
The section highlighted in green is all you need to add. You have two options here:
- auto - this enables the app to be installed on the SD card, but you don't have a preference and you leave it up to the user. This is the option I always go for. Once installed the app can freely move between internal and external.
- preferExternal - this also enables the app to be installed on the SD card but you also try and install it there by default if there is available space. If there isn't it will default to internal phone storage. Once installed the app can freely move between internal and external.
That is all there is to it - a one line change which makes a massive difference to your user retention rates. The full documentation can be found here.