No, you generally do
not need to have the car running when adding refrigerant (freon) to a Ford Focus's air conditioning system. In fact, it's often recommended to have the car off. The process is usually done with the system under vacuum to remove moisture and air before adding refrigerant. Running the engine complicates this process and could be dangerous.
However, after adding the refrigerant, you'll need to run the engine and AC to check for leaks and proper operation. Always consult your vehicle's owner's manual or a professional mechanic for specific instructions.
