xdm is probably the display manager I have used less - so I have no idea on how to do that.
But you can autologin using most displaymanagers lxdm, lightdm, sddm.
No matter how you flip the coin the system will need a username.
If you do console you will have to provide username and password - then run startx.
If you do display manager you can preload the config with a username which then will be used to start the graphical system.
The backdraw to doing that is that no keyrings are unlocked because that is impossible without password so - at some point - you will be asked for authentication.
Executing any task as superuser - will still require authentication - there is no way of turning Linux into Windows XP.
On Arch wiki read XDM - ArchWiki
In order to enable passwordless login for XDM, add the line below to