I add a ball valve to all my cars that don't have a valve from the factory.

Get a ball valve for 3/8" threaded pipe and a 6" ish long piece of 3/8" black steel pipe. Cut the pipe in half , debur the ends and thread it into the valve. 3/8" threaded pipe is slightly larger than 5/8", so 5/8" hose will stretch over and 3/4 will clamp down.

Install the valve between where the hot water leaves the engine and it enters the heater core. Some OE systems shut off the water as it leaves the heater core so poke around a bit to make sure.