Swift - POST request, sending JSON with Vapor 3 - json

Question

I have a problem sending a POST request with Vapor 3 with the body including JSON. I am using https://docs.postman-echo.com/ to test this, where it responds back with the same JSON it is sent.
I have viewed answers on here, but got errors with encoding and content types.
router.get("hooray") { req -> Future<View> in
var postHeaders: HTTPHeaders = .init()
postHeaders.add(name: .contentType, value: "application/json")
postHeaders.add(name: .accept, value: "*/*")
postHeaders.add(name: .acceptEncoding, value: "gzip, deflate")
let oneField = singleGet(foo: "barfoobar")
// { foo: "barfoobar" } - JSON string
let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let jsonData = try encoder.encode(oneField)
let jsonString = String(data: jsonData, encoding: .utf8)!
let postBody = HTTPBody(string: jsonString)
let httpReq = HTTPRequest(method: .POST, url: "/post")
let httpRes = HTTPClient.connect(hostname: "postman-echo.com", on: req)
.flatMap(to: singleGet.self) { client in
req.http.headers = postHeaders
req.http.contentType = .json
req.http.body = postBody
return client.send(httpReq).flatMap(to: singleGet.self) { res in
return try req.content.decode(singleGet.self)
}
}
return try req.view().render("test", httpRes)
}
struct singleGet: Content {
let foo: String
}
I am getting the correct response with this code, but I was wondering when I rework the code to match this answer, why do I get errors?
I've added the headers and body to the request and comment them out inside the closure, but it won't work this way.
let httpReq = HTTPRequest(method: .POST, url: "/post", headers: postHeaders, body: postBody)