I'm not sure if there's any ready-made device that can do this more easily and elegantly.
But this is how I would do it. It's probably not the best way to do this.
I'd get a Raspberry Pi board that's powered by a 5v power supply. The power supply would be connected to the power of the flood light, assuming it's just a regular AC plug. If it's a standard light bulb socket, I'd get one of these that adds an extra AC outlet to a light bulb socket.
https://www.amazon.com/Light-Bulb-E26-Socket-Adapter-Polarized-Converter-Standard/dp/B07HQ918GLWhen the flood light turns on, the Pi also turns on. Then on the Pi, I'd write a simple script to send an email or some other notification process to wherever you want to be notified. The script will only run once when the Pi first starts up (when it gets power). When the flood light is off, the Pi is off. If the flood light turns on again, the Pi will run the script again. I suppose if you have an extra computer, you can do the same thing but it would just take up a lot more space.