Repro attempt https://stackoverflow.com/questions/52259616/flurl-posturlencoded-does-get-instead-of-post

Test.cs

[Test]
public async Task StackOverflowQuestion() {
    var message = "";
    try {
        var cli = new FlurlClient("https://www.google.com").EnableCookies();
        var s = await cli.Request().GetStringAsync();
        var result = await cli.Request("foo").PostUrlEncodedAsync(new { x = "x" });
    }
    catch (FlurlHttpException ex) {
        message = ex.Message;
    }
    Assert.AreEqual("Call failed with status code 404 (Not Found): POST https://www.google.com/foo", message);
}