I suspect you'll need a powered otg cable (where there's an extra dongle for you to plug in a charger) as the device is unlikely to supply the current needed to power the keyboard. I don't have one myself so I can't recommend one very effectively. Please do post back if you find something that works for you.
My concern about which one to buy is that you get a cable that allows you to power the peripheral rather than merely charge the ereader at the same time you use a peripheral. I have no idea if there are different subsets of otg cables that handle where the power goes differently, but perhaps to be on the safe side, you could get a powered usb hub (which is designed to power peripherals) plug your keyboard into that and then plug the hub into your current otg cable.
Good luck
