Hello. When I'm turning in my PC, I firstly see the screen where I can choose a user. But I don't need. Is there a way to specify a default user and boot right into a window where I can paste my password?

Hello!

This depends on the SDDM-Theme you chose and the distribution you are running.
I have the last user logged in active and I only have to type the password using Breeze-Theme.

See if your system has the file /etc/sddm.conf.d/kde_settings.conf
It should contain a section called "[autologin]"

mine looks like this:

[Autologin]
Relogin=false
Session=plasma
User=hannahk

obviously you have to replace hannahk with your username