How do I season an iron griddle on an induction hob?
You don't. 

I've tried to do this. It doesn't work. 

The problem is that the induction hob heats a cast iron pan to very hot temperatures, even on the lowest setting. And when it doesn this, it doesn't heat evenly, you get a coil-shaped hot spot. 

We have had a question about seasoning cast iron on stovetop, and somebody reported good results provided that it is done "low and slow". I can imagine this working. But on the induction, there is no way to go low and slow. The pan gets too hot after half an hour at the latest, the oil burns on the hot spot and stays liquid outside of it. 

The easiest version might be to season it at somebody else's place (parents, friends) and then use it normally at home. Once the seasoning is done, there is no problem with that. I haven't done that; I started seasoning in my toaster oven. Even when the pan is too large to close it, the results are still better than on the induction hob.