She pretended to be indifferent toward him on purpose.
So this doesn't quite make sense in terms of the meaning of the kanji... but you'll be okay. This means on purpose. Perhaps when you do something that has the appearance of doing what you wanted to do you're doing it on purpose.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you: "You did that to me on purpose? Whazapp with that?"