"I wash my hands before a meal."
「しょくじのまえにてをあらいます/食事の前に手を洗います」 should be accepted. Reported on Oct. 31, 2017.
A year since your comment, I've just answered with 食事の前に手を洗う and it was marked wrong today (29th Oct. 2018).
Now I'm wondering if your reported sentence has been added yet. Will try your one if I see this sentence again.
Pretty sure the に doesn't need to be included for the sentence to be grammatically correct.