The easiest way to accomplish this would be to add a website shortcut to the Startup folder. Please see below:
Win+Rto 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!