//Arrange
var request = new Mock();
request.Setup(x => x.Scheme).Returns("http");
request.Setup(x => x.Host).Returns(HostString.FromUriComponent("http://localhost:8080"));
request.Setup(x => x.PathBase).Returns(PathString.FromUriComponent("/api"));

var httpContext = Mock.Of(_ => 
    _.Request == request.Object
);

//Controller needs a controller context 
var controllerContext = new ControllerContext() {
    HttpContext = httpContext,
};
//assign context to controller
var controller = new SomeController(){
    ControllerContext = controllerContext,
};

String expected = "expected value here";

//Act
String actual = await controller.SomeMethodPost();


//Assert
Assert.AreEqual(expected, actual);

//...