It was explained by @lubos at Custom domain - #2 by lubos that this is not possible unless you use reverse proxy but you could also use the less pervasive ???.accounting.link:
… if your login URL is
something.manager.iothensomething.accounting.linkwill work too…
Truth of the matter is that I do not think one should pass on an application as made by your own company and just do the reverse and explain that you are part of a global well known application that you are providing support to your customer for. It is unethical in my view to hide the intellectual property. Especially in accounting, trust and transparency, are key to successful business development and investment.