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Configure the Enterprise Mode Site List. With the help of this policy, all intranet sites will be loaded in the IE mode in the MS Edge browser. Send all intranet sites to Internet Explorer. I suggest you enabled this policy and set its value to Internet Explorer mode.
Configure Internet Explorer integration. You can configure IE mode by setting a group polices for the MS Edge browser. It uses the integrated Chromium engine for modern sites, and it uses the Trident MSHTML engine from Internet Explorer 11 (IE11) for legacy sites. IE mode on Microsoft Edge makes it easy to use all of the sites your organization needs in a single browser. MS Edge Chromium browser comes with the IE mode feature.
You can do it from the Edge browser side.
If you are looking for a solution to apply in your code to load the IE 8 mode in the Edge browser then I did not get any way to do it.