This question already has an answer here:
- Can I write notes in my passport? 2 answers
Electronic Travel Authorization (eTA) of Canada is transferred as a separate sheet-of-paper document that contains basically one number and the valid date. Can I, instead of keeping it with me for the next 5 years, write the eTA number in my passport? Will it be accepted by the border officers/airlines?
Note that this question is different from the one it is marked a duplicate of, as eta number is being asked by airlines as travel authorization, so it makes some sense to have it in your passport (comparing to random private notes).