Which key is for flash?

Thats like asking someone what hand you are suppose to throw a baseball with. Its preference

I use Q and E for my summoners. E is always for flash. I use WASD for my abilities instead of QWER. (Don't try this out if you're already used to QWER, I just changed to it during the first game I played league because it felt more natural for me as WASD are the common navigation keys on other games.)

But Jimmy, I don't have a keyboard!

Flash? What is this summoner spell you speak of? It goes in neither of my slots thank you very much.

depends on what other summoner spells are...