Computer Configuration > Administrative Templates > Microsoft Edge > Startup, home page and new tab page
Both
HKLM\Software\Policies\Microsoft\Edge
This policy only works if you set the 'RestoreOnStartup' (Action to take on Microsoft Edge startup) policy to 'Open a list of URLs' (4) and the 'RestoreOnStartupURLs' (Sites to open when the browser starts) policy as mandatory. If you enable this policy, users are allowed to add and remove their own URLs to open when starting Edge while maintaining the admin specified mandatory list of sites specified by setting 'RestoreOnStartup' policy to open a list of URLS and providing the list of sites in the 'RestoreOnStartupURLs' policy. If you disable or don't configure this policy, there is no change to how the 'RestoreOnStartup' and 'RestoreOnStartupURLs' policies work.