Responses are not meant as a substitute for professional advice. Answers are intended as outline only the advice of a qualified professional with access to all relevant information should be sought before acting on any response given.
I would get the from signed, but not send it to HMRC, just keep it for your records, doing the application bit online. It's usually much quicker, unless of course your client loses the authentication code!
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Joanne
Winner of Bookkeeper of the Year 2015, 2016 & 2017
Thoughts are my own/not to be regarded as official advice,which should be sought from a suitably qualified Accountant.
You should check out answers with reference to the legal position