Ciao,
io non ho ancora provato; ma ho trovato questo:
1) Inside runtastic under the Activity tab, go to Settings -> Heart Rate monitor -> Connect Heart Rate monitor and press Connect Now
2) On the Mi band perform a heart rate check
3) It should connect you to the Runtastic app, it doesn't always work on the first try but keep on trying it is a one time process
3) Before your run turn on exercise mode on the band, for the continuous HR tracking during the run.