To be precise, 안녕하살법 받아치기 is correct thing. this is meme in korea but it also originated from japanese animation《카구야 님은 고백받고 싶어 ~천재들의 연애 두뇌전~》(かぐや様は告らせたい〜天才たちの恋愛頭脳戦〜). In this animation.
안녕하살법 (this means maybe 'Hello-Murder technique' maybe but there is no meaning acutally)
and 안녕하살법 받아치기 (this means maybe 'Hello-murder technique reaction, but there is no meaning acutally too)
if someone says "안녕하살법(こんにち殺法)", the other person responds with "안녕하살법 받아치기(こんにち殺法返し)"
That's the way just saying hello with fun