Hey Timo!
The easiest way to accomplish this would be to add a website shortcut to the Startup folder. Please see below:
- Hold
Win+R
to open the Run box & enter *Shell:Startup
*to open the Startup folder. - Open a web browser and head to the site in question.
- Drag the padlock icon of the site into the Startup folder
-
This is for the local user only, though. If you'd rather this apply to the entire device, regardless of who is logged in, head to %ProgramData%\Microsoft\Windows\Start Menu\Programs\Startup for #2.
Let me know if that works. Thanks!