Adding a Flow Regulator to an ecosmart Electric Tankless Water Heater

How I added a flow regulator to an ecosmart ECO 27 electric tankless water heater.
After installing the heater, I was receiving an E5 error code when running the bathtub. I purchased a flow regulator to limit the water output and avoid lukewarm bath water.
Flow regulator: www.supplyhous... (check your manual to make sure you buy the right size)
Water heater: www.amazon.com...