If i have a Forbidden Technique in my trash. Can i use my Kaisa (legend) to pay one power cost of Forbidden Technique after playing a cost 7 card?

Yes. Once you've played a card with Energy cost 7 or more, Forbidden Technique can be played from your trash for its alternate cost of [1] plus two power, and Kai'Sa, Daughter of the Void can pay one (or both) of those power symbols — her ability adds power of any domain usable only to play spells, and this is playing a spell.

Timing-wise, because her ability has [Reaction] and adds resources, you can exhaust her during the pay-costs step even though the chain is closed (Rule 357.1.a).

One thing to watch: the "7 or more" check uses the card's printed Energy cost, not what you actually paid (Rule 206).