Unit test failing while mocking async method, c#

I have mocked async function, but, whenever I am invoking mocked function, it is returning null value.

I have an async function as shown below

public async Task<TResult> InvokeAzureFunctionWithTokenGetResultAsync<TArg, TResult>(TArg arg, string functionUri, string resourceid, CancellationToken canceltoken)

I want to write xUnit test case, as I cannot execute actual function, I am trying to mock it up. when executed there is no error, but when the function is invoked, I am getting null.

Actual function

public async Task<TResult> InvokeAzureFunctionWithTokenGetResultAsync<TArg, TResult>(TArg arg, string functionUri, string resourceid, CancellationToken canceltoken)
{
    // some external code here

    if (message != null && (message.StatusCode == HttpStatusCode.OK || message.StatusCode == HttpStatusCode.Created))
    {
        try
        {
            return JsonConvert.DeserializeObject<TResult>(await message.Content.ReadAsStringAsync().ConfigureAwait(true));
        }
        catch (JsonReaderException re)
        {
            throw new FormatException(re.Message, re);
        }
        catch (FormatException fe)
        {
            throw new FormatException(fe.Message, fe);
        }
    }
    else if (message != null)
    {
        throw new FormatException(message.ReasonPhrase);
    }

    return default(TResult);
}

Test function

IHttpEndpointInvoker httpEndpointInvoker = new HttpEndpointInvoker();

IHttpEndpointInvoker httpEndpointInvoker = new HttpEndpointInvoker();

try
{
    var mock = new Mock<IHttpEndpointInvoker>();

    var obj = (new RouteDefinition() { Id = routeDefinitionId, IoTHubId = "" });

    mock.Setup(s => s.InvokeAzureFunctionWithTokenGetResultAsync<RouteDefinition, string>(It.IsIn<RouteDefinition>(), It.IsAny<string>(), It.IsAny<string>(), new CancellationTokenSource().Token)).Returns(Task.Factory.StartNew(() => GetHttpResponseMessage(HttpStatusCode.Accepted).Result));

    var o1 = mock.Object.InvokeAzureFunctionWithTokenGetResultAsync<RouteDefinition, string>(obj, "functionUri", "resourceid", new CancellationTokenSource().Token).GetAwaiter().GetResult(); // this line is returning null. But I am expecting value that is returned by "GetHttpResponseMessage" function


}
catch (Exception ex)
{

    throw;
}

Note: "GetHttpResponseMessage" is successfully called and returning a string. I have debugged it.

Please check the line with below comments given in the above code for expected output/results.

// this line is returning null. But I am expecting value that is returned by "GetHttpResponseMessage" function