All in all it is about adding the command for starting the programm to
sudo nano /etc/xdg/lxsession/LXDE-pi/autostart
the command, that worked for me is adding
@/usr/bin/chromium-browser --kiosk --disable-restore-session-state www.domain.com
I am only using the Pi over Remote Desktop and when I reboot and connect I can see Chromium in the Taskmanager, but not on the Desktop. I also know, that it works, because I used a website from which I get notified when I visit it, but when I connect to the Pi via Remote I can not see the Chromium Window. Is there any way to not opening a new session or something?
I am really new to Linux and help would be appreciated :)