Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
Define a list of sites, based on URL patterns, that are not subject to the 'EdgeSidebarAppUrlHostBlockList' (Control which apps cannot be opened in Microsoft Edge sidebar). If you don't configure this policy, a user can open any app in sidebar except the urls listed in 'EdgeSidebarAppUrlHostBlockList'. If you configure this policy, the apps listed in the allow list could be opened in sidebar even if they are listed in the block list. By default, all apps are allowed. However, if you prohibited apps by policy, you can use the list of allowed apps to change that policy. For detailed information about valid url patterns, see https://go.microsoft.com/fwlink/?linkid=2281313. Example value: https://www.contoso.com [*.]contoso.edu