This post shows you how to replace Request.IsAjaxRequest() in ASP.NET Core.

You can't find Request.IsAjaxRequest() method in ASP.NET Core.

To check your ajax request in ASP.NET Core you can use Context.Request.Headers.

var ajaxRequest = Context.Request.Headers["X-Requested-With"] == "XMLHttpRequest";

I hope so, you can solve your problem.