Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
This policy lets sites configured to open in Internet Explorer mode to be opened by Microsoft Edge for testing on a modern browser without removing them from the site list. Users can configure this setting in the "More tools" menu by selecting 'Open sites in Microsoft Edge'. If you enable this policy, the option to 'Open sites in Microsoft Edge' will be visible under "More tools". Users use this option to test IE mode sites on a modern browser. If you disable or don't configure this policy, users can't see the option 'Open in Microsoft Edge' under the "More tools" menu. However, users can access this menu option with the --ie-mode-test flag.