Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
OBSOLETE: This policy is obsolete and doesn't work after Microsoft Edge 99. This policy is obsolete because it was only intended to be a short-term mechanism to give enterprises more time to update their web content if and when it was found to be incompatible with the change to disallow synchronous XHR requests during page dismissal. It doesn't work in Microsoft Edge after version 99. This policy lets you specify that a page can send synchronous XHR requests during page dismissal. If you enable this policy, pages can send synchronous XHR requests during page dismissal. If you disable this policy or don't configure this policy, pages aren't allowed to send synchronous XHR requests during page dismissal.