Right there is no login page as such. The login is available on all pages and only when attempting to carry out an action that a Guest has no privileges to use is a login form presented in the middle of the page - and this is done via an error message template.
So you would have to do something like this,
https://vborg.vbsupport.ru/showthread.php?t=222607 of course that is a old article, but it will give you the idea.