We don't currently support custom domain – your users will always see honey.is when signed into their account.
Alternatively, your best option here is to work with your IT or technical team to set up the URL you'd like to use as a redirect to our sign in page (https://honey.is/signin). Once signed in, users will see the https://honey.is URL, as we don't support custom domains in-product, but the re-direct will make it super easy for your team members to remember and access the sign in page quickly.
We have lots of teams to do this and it works wonderfully. For example, at Sesame Street, all employees use http://honey.sesame.org to sign in to Honey, as it redirects to our sign in page. This is also the URL that their IT team bookmarked on all employee's browsers and their internal comms team uses as an official communication because it's Sesame-branded.