A follow-up to https://github.com/backdrop/backdrop-issues/issues/3108
I'd really like to redesign the log-in page to remove the "tabs" entirely. The visual "tabs" are a strange way to navigate these three pages. Plus, they really give away that a site is Drupal/Backdrop.
I almost always remove these tabs in my custom themes, and replace them with a markup element in the form containing links to those pages instead. I'd love to see us do that by default for all Backdrop sites.
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For such cases I like EVA https://backdropcms.org/project/eva
Display field using content of another field
If you don't want/need a View, I think Computed Field would be an excellent option for this. BTW, you can use token replacement (by using the function token_replace()) in the computation of the field...
Display field using content of another field
Views would be my first port of call, but I guess you've already thought of that. I haven't used it but you could also look at Views Field Formatter
Display field using content of another field