Thank you this helped me figure it out. I assumed the data type would be sent serialized through Retrofit. I am testing this on a local server so I do have access. I needed to update the server PHP script. Below is what I needed to do to receive the data.

<?php

....
$data = json_decode(file_get_contents('php://input'));

$username = $data->{'username'};
$email = $data->{'email'};
$password = $data->{'password'};

....
?>