Copy and paste the following URL into the File > Preferences > "Additional Boards Manager" textbox.

https://raw.githubusercontent.com/StackRduino/StackRduino_M0/master/Arduino/package_StackRduino_index.json

Make sure to also install the Arduino SAMD Library at the same time.

Boards > "Add board definition" > Search for "StackRduino Boards" > Install Robo HAT Library

Boards > "Add board definition" > Search for "SAMD" > Install Arduino SAMD Library