There are several brands of STOP LEAK so get one and follow directions on label. Not only for radiators but heater core to.
The more expensive ones seem to work better. Make sure the presure cap is good because it needs to get hot and build up pressure for the stop leak to be pushed in the leaking hole. Anti freeze doesn't help but you can't do without it now. No other way to fix it right but tear out the dash and replace the core. Possiblity the hose could be leaking & tighten the clamp a little. You'll have no heat but clamp off the water out & in lines on the engine and take a warm blanket. Or run a hose from the out connection on the engine to the return and by pass the heater.