MK2i Air Integration link Senders to Bottom Gas

Hi

how Do I link a transmitter to the bottom Gas?

In the garmin Dive App I see the data under the Tank Sensors section but in the main screen, where the bottom Gas is shown the values are always empty cause the garmin does not know what sensor is linked to what gas. 
but there mus be an option to create that link. Otherwise this Bottom Gas tank value screen does not make sense!!??

Anyone knows?

regards

david