I already made it to enter some additional text to the
mydomain/wiki/Special:UserLogin.
I added a phrase:
In order to apply for an account send an mail to…
But I have absolutely no clue anymore how I did that.
All the hints in the web apply to modify the standard text blocks by adding
?uselang=qqx to the URL to see the appropriate MediaWiki pages...
But how did I make it to add this text to the Special:UserLogin page?