var result = await "https://api.mysite.com"
    .AppendPathSegment("person")
    .SetQueryParams(new { api_key = "xyz" })
    .WithOAuthBearerToken("my_oauth_token")
    .PostJsonAsync(new { first_name = firstName, last_name = lastName })
    .ReceiveJson<T>();

[Test]
public void Can_Create_Person() {
    // fake & record all http calls in the test subject
    using (var httpTest = new HttpTest()) {
        // arrange
        httpTest.RespondWith("OK", 200);

        // act
        await sut.CreatePersonAsync("Claire", "Underwood");
        
        // assert
        httpTest.ShouldHaveCalled("http://api.mysite.com/*")
            .WithVerb(HttpMethod.Post)
            .WithContentType("application/json");
    }
}