Make a small hole in the form, right in the center where the box will be. Take a length of ty wire and pull the box (from the inside) against the inside of the form. Twist the ty wire over a piece of wood or a small piece of conduit. Make it good and tight. This will hold it from the outside.
Note: Remove the screws from the box. So it lays flat. Put a dab of grease in the screw holes, or put tape over them. Tape the box with masking tape to keep the concrete from entering. Some will enter through the front where it meets the form. This is okay. It will be very thin with no rock. Make sure you put some tape over the connectors, so none can enter the conduit. When the form comes down, you may need to chip it out if the form does not clear it for you first.