Hello and welcome to FF ! Please read this care guide: Frog Forum - Pacman and Horned Frogs - Ceratophrys - Care and Breeding. As previously stated by others, you can place heater on side. Just be aware they are not very efficient used that way.

My preferred heating method is to use a 60W Ceramic Heat Emitter with an in-line dimmer controlled dome (made by Fluker's and others). Cover your screen top with aluminum foil, place lights on top and draw around them with marker, cut hole(s) open with scissors around 1 in. larger perimeter and you are done. Foil helps lot's to keep humidity and heat in.