You don't need to indicate a break in your logbook. The BIT app automatically starts calculating your break when you go into a non-driving duty status.
To take your required 30-minute break simply set your duty status to Off Duty, Sleeper Berth, or On Duty Not Driving, and stay in it for 30 minutes or more.
Your Available Hours clocks will show the Break clock as "Break remaining" and count down from 30 minutes. When your clock shows zero minutes remaining, you have completed your 30-minute break requirement and can start driving again.