Computer Configuration > Administrative Templates > Microsoft Edge
Both
HKLM\Software\Policies\Microsoft\Edge
The Origin-Agent-Cluster: HTTP header controls whether a document is isolated in an origin-keyed agent cluster or in a site-keyed agent cluster. This has security implications because an origin-keyed agent cluster allows isolating documents by origin. The consequence of this for developers is that the document.domain accessor can no longer be set when origin-keyed agent clustering is enabled. If you enable or don't configure this policy, documents without the Origin-Agent-Cluster: header will be assigned to origin-keyed agent clustering by default. On these documents, the document.domain accessor will not be settable. If you disable this policy, documents without the Origin-Agent-Cluster: header will be assigned to site-keyed agent clusters by default. On these documents, the document.domain accessor will be settable. See https://go.microsoft.com/fwlink/?linkid=2191896 for additional details.