Here's how I did a POST request that worked:
var url: String = "http://example.com/script.php"
var query: String = "param=value"
var headers: Array = [
"Content-Type: application/x-www-form-urlencoded;charset=utf8",
"Content-Length: %d" % len(query)
]
var error: int = http_request.request(url, PoolStringArray(headers), true,
HTTPClient.METHOD_POST, query)