Computer Configuration > Administrative Templates > Microsoft Edge > Startup, home page and new tab page
Both
HKLM\Software\Policies\Microsoft\Edge
Configures the default URL for the new tab page. The recommended version of this policy doesn't currently work and functions exactly like the mandatory version. This policy determines the page that's opened when new tabs are created (including when new windows are opened). It also affects the startup page if that's set to open to the new tab page. This policy doesn't determine which page opens on startup; that's controlled by the 'RestoreOnStartup' (Action to take on Microsoft Edge startup) policy. It also doesn't affect the home page if that's set to open to the new tab page. If you don't configure this policy, the default new tab page is used. If you configure this policy *and* the 'NewTabPageSetFeedType' (Configure the Microsoft Edge new tab page experience) policy, this policy has precedence. If a blank tab is preferred, "about:blank" is the correct URL to use, not "about://blank". This policy is available only on Windows instances that are joined to a Microsoft Active Directory domain, joined to Microsoft Azure Active Directory, or instances that enrolled for device management. On macOS, this policy is available only on instances that are managed via MDM or joined to a domain via MCX. Example value: https://www.fabrikam.com