Is it acceptable to write the electronic Travel Authorization in a free page of your passport? [duplicate]

As one of the previous answers indicates, writing in a passport besides in a place for a signature or return address could create issues.

eTAs are registered in the Canadian immigration system/Public Information Bank, and should not be carried around.

They are not visas, or electronic visas, as they are issued to nationals of visa-exempt countries.

There is no reason for one to carry it around unless you have it recently issued. I remember speaking to travellers who had "issues" with eTAs when arriving in Canada during the grace period.

In all of those cases, the eTA was recently issued, within the last week. It would seem it takes some time for it to register in the Canadian immigration system. The travellers had it printed, and carried it in addition to their papers for car rentals, tours, and so on.

One would assume that you have applied for an eTA well in advance of your travels, as some approvals extend past the several hour window for automatic approval.

http://www.cic.gc.ca/english/visit/eta-facts-en.asp

No, no, just no

You do not need to carry the eTA in any form, only the passport, because the eTA appears when check-in staff and Canadian border guards scan your passport.

Furthermore, passport amendments are for authorities to make, not you. In theory you doing it may well render the passport invalid in the eyes of border authorities.

If you want to play it really safe, put the number on a post-it note