The run dialog accepts not only a command but also a URL/URI or any real paths, as you can see from its description:

That's why when you open shell:startup
which is a special path (specified in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\explorer\FolderDescriptions
) it works normally
However in Windows cmd or PowerShell the command must be an executable file, a script file or any other files with a registered type. It won't accept a folder. To work around that use either of the following
start shell:startup
explorer shell:startup
If the path contains spaces then in cmd you must add another empty parameter due to the legacy issues in cmd.exe
start "" "shell:common startup"
If you use PowerShell then start "shell:common startup"
will work normally because start
is an alias to Start-Process
in PowerShell. If you run explorer
then you don't need that empty string either