I'd look up on google the numerous guides to writing motivational letters/cover letters for airline pilots. There isn't much you can do wrong, in my opinion, especially with how short (isn't it like 250 words?) the requirement is - especially as it's not asking for you to talk about yourself. I think it's mainly there to weed out people with really bad (or no) English, so make sure your spelling and grammar are fine. Pro tip - use something like grammarly to spot any obvious errors and, of course, get someone else to proofread it as well if you're worried about it.