Hello,
we are using KDM on our lab workstations for our students (~50 new accounts per term) to log in to KDE sessions. At the beginning of a term students have to sign up for a new account using a web based user registration system. Being told a username and a password beforehand – up to now they have to log into a guest account, start a browser and type the URL of the aforementioned registration system. I know the latter part could be automated easily but we're looking for something more fool-proof...

Is there a way (without having to patch and recompile KDM) to have KDM show an additional button that could be utilized e.g. for a one-click login or the like?

Thanks in advance!
Best
Thomas

Run "kcmshell4 kdm" (in doubt, run "kdesu kcmshell4 kdm")
In the last tab preselect that user ("guest"?) and allow passwortless login for that user.

-> All the students will have to do is to press the login button (or hit enter)

(Although I really don't understand that process - you could hand them their account with a pregenerated pssword and tell them to change the password within the next week or it will expire)

Ok, thanks so far. That already helped me quite a bit. But after all another question came up in this context:

I know that in /usr/share/xsessions i can (and already have done so) make a new session type e.g. "Sign up" that runs a browser showing our sign up form. (Without any window manager.)
However I'm wondering whether or how I can restrict the session type for a certain user ('signup') to a certain value like for instance the one I've created. In other words: Is there a way to hide all Session-Types like KDE-Plasma, Gnome, etc. in KDM but only show my 'SignUpSession' for a certain user (e.g. signup)?

Not that i'm aware of (also notice that this would not provide any security additions - with some unspecialized GUI client running you'll likely always be able to get full shell access)

You can edit the guests ~/.dmrc to have "Session=register_student" and make it immutable (chattr +i /home/guest/.dmrc), but that would only ensure them to have the "right" session as default, not prevent them from logging into KDE etc.