This and the interval of time between when the finger is touched to the screen and finger movement. So an immediate diagonal movement after touching the screen should start the drawing of the box. If you press and hold too long you may see the gimbal tilt indicator appear, in which case just lift your finger and try again.
If you still don;t have the magic touch to select the target, you can try enabling "Target Scanning" and the Fly app will identify and highlight targets you can track.
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