Final answer:
To input text into an existing FHP CAD entry, the Corrected Case command 'CC' must be used. This ensures that the system recognizes modifications to an existing record.
Step-by-step explanation:
The command that must be entered before inputting text into an existing FHP (Florida Highway Patrol) Computer Aided Dispatch (CAD) entry is CC (Corrected Case).
This command allows a dispatcher or operator to make corrections or add information to an existing CAD record. Before entering text into the system, one must use this command to ensure the system recognizes that the following text is meant to modify an existing entry rather than creating a new one.