App? Tax?
I don't like to see the two words in the same sentence. Something to do with security risk?
Anyway, it's not needed. HMRC are already digitally aware - I regularly get e-mails from them telling me that I'm due a tax refund and click on the link below to claim. Of course, they could spend a few quid tidying up their spelling - sometimes they're HM-RC@internalrevenue.gov.uk, sometimes they're H-M-R-C@hmrc.g0v.uk