Hello,
When I log onto the router itself, the login form is prefilled with the user ‘admin’.
I’d like the ‘Login’ field empty by default (see screenshot 1).
The prefill could be helpful, but it’s not in my case, because I disabled the admin account and created another one for admin purposes.
Looking at the HTML source, it’s prefilled from the code served by the router (see screenshot 2).
Is it possible to make the ‘Login’ field empty by default?
Thank you in advance.
No, you can’t, but a better option is to not use Web config, but use Winbox instead. https://mt.lv/winbox
Isn’t that an option available in “branding”? 
Branding can’t change webfig, only the pre-login page.
Isn’t the OP asking about what you call the pre-login page and that is called “Default webpage” on the help page about branding and that (loosely) looks like the picture here:
https://help.mikrotik.com/docs/display/ROS/Webfig
https://help.mikrotik.com/docs/download/attachments/328131/webfig.png?version=3&modificationDate=1571210992820&api=v2
yes, my bad, but in any case, use winbox 
I see, here are the two recommended options to access a Mikrotik router:
Thanks. I’ll give it a closer look out of curiosity, but at first sight, it looks over-complicated for a small problem. I might as well use a Greasemonkey script in the browser.
Winbox and Webfig have the same amount of menus. Winbox is simply more stable, faster and works even if there is no IP. So it is not more complicated, it is only more reliable.
Personally I think that personalization of the web interface through branding and skin, together with a sensible user management is a useful resource.
I can imagine many use cases when you might want to give to some users access to the router with a simplified menu and without having them download and run winbox.
I know it is not wise from a security viewpoint, still it may be useful, though the procedure seems to be particularly complex (for “branding”), surely it is not worth it for such a minor nuisance as the pre-filled “admin”.
Try to paste large script to Winbox GUI script form field… Had to use WebFIg for this.
Also Winbox crashes when trying to open large script (35KB), Winbox64 running with wine-8.0.1 (WineskinCX 23.7.1), attached log.
winbox_crash.log (4.83 KB)