you beat me to it.
but this assumes you are using a midi instrument.
If it is audio data, sorry there is no dynamic option builtin in Ableton (that I know of, I have Standard edition), but instead you would resort to a third party octaver like from MeldaProduction's, MHarmonizer:
the above example gives you a power chord, however the notes in green can come from a midi source as well, like what you can generate from the FCB1010 foot controller. The Mharmonizer is not that expensive either and I found it to be one of the best. I use it for my electric violin.