僕と契約して魔法中年になってよ Boku to keiyaku shite mahō chūnen ni natte yo
“Make a contract with me and become a magical middle-aged person”
Of note is Sakino demonstrating “Ryakuji (Japanese: 略字 "abbreviated characters", or 筆写略字 hissha ryakuji, meaning "handwritten abbreviated characters") are colloquial simplifications of kanji”
魔法 = Mahō = Magic 魔 → He kept that outside bracket of the kanji, but replaced all of the inner strokes with the Katakana “ma” 魔 → マ (since “ma” is also the reading of the kanji)
Make a contract with me and become a magical middle-aged person.