Chunin exams definitely ! In my au they meet in the chunin exams, but Haku doesn’t actually compete since I don’t think they’d just let a random former rogue ninja with no village affiliation compete. Their interaction would be pretty brief, Haku just notes to herself how strange he looks even though she feels bad about thinking so while Lee thinks she’s very pretty but doesn’t say anything and also he’s still crushing on Sakura ofc. I dont see them interacting as more than acquaintances until the Sasuke retrieval arc where Lee comes in to save Haku (and Naruto) from Kimimaro. That’s when I like to see Haku developing a crush on Lee even though she doesn’t notice it till way later haha.