1) Ensure official Mi Fit app is installed and paired correctly with your band
2) Connect your phone to the computer
3) Ensure adb is installed and working correctly (google adb install for more instructions)
4) Open console command line on your computer
5) Execute command: adb shell
6) Execute command: run-as com.xiaomi.hm.health
7) Execute command: cd data/data/com.xiaomi.hm.health/databases/
8) Execute command: sqlite3 origin_db_ "select AUTHKEY from DEVICE"
Now you have got your Auth key code