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("Frank", "Reynolds");
        
    // assert
    httpTest.ShouldHaveCalled("http://api.mysite.com/*")
        .WithVerb(HttpMethod.Post)
        .WithContentType("application/json");
}