Why url encoding does not work in TypeScript / fetch?

Question

In Swift, this API call returns an invoiceId

return client.post("https://www.szamlazz.hu/szamla/", headers: HTTPHeaders([("Content-Type", "application/json")]), beforeSend: { (req) in
    try req.content.encode(["action-xmlagentxmlfile": xml], as: .urlEncodedForm)
}).flatMap { (response) -> EventLoopFuture<Void> in
    if response.status == .ok && !response.headers.contains(name: "szlahu_error") {
        if vegszamla != true {
            p.invoiceCreatedAt = Date()
            p.invoiceId = response.headers["szlahu_szamlaszam"].first

but the similar one in TypeScript return null, why? Same content is sent.

const formData = new URLSearchParams()
formData.append('action-xmlagentxmlfile', xml)
const response = await fetch(`https://www.szamlazz.hu/szamla`, {
    method: 'POST',
    body: formData.toString(),
    headers: {
        'Content-Type': 'application/x-www-form-urlencoded',
    },
})
if (
    response.status === 200 &&
    !response.headers.has('szlahu_error')
) {
    const invoiceId = response.headers.get('szlahu_szamlaszam')
    if (!invoiceId) {

No error raised.

---

I tried application/json as content type, same result.
I see this as response:

Response {
  [Symbol(realm)]: null,
  [Symbol(state)]: {
    aborted: false,
    rangeRequested: false,
    timingAllowPassed: true,
    requestIncludesCredentials: true,
    type: 'default',
    status: 200,
    timingInfo: {
      startTime: 6905.054625034332,
      redirectStartTime: 6905.054625034332,
      redirectEndTime: 6966.9528748989105,
      postRedirectStartTime: 6966.9528748989105,
      finalServiceWorkerStartTime: 0,
      finalNetworkResponseStartTime: 0,
      finalNetworkRequestStartTime: 0,
      endTime: 0,
      encodedBodySize: 335,
      decodedBodySize: 0,
      finalConnectionTimingInfo: null
    },
    cacheState: '',
    statusText: 'OK',
    headersList: HeadersList {
      cookies: null,
      [Symbol(headers map)]: [Map],
      [Symbol(headers map sorted)]: null
    },
    urlList: [ [URL], [URL] ],
    body: { stream: undefined }
  },
  [Symbol(headers)]: HeadersList {
    cookies: null,
    [Symbol(headers map)]: Map(12) {
      'date' => [Object],
      'server' => [Object],
      'set-cookie' => [Object],
      'cache-control' => [Object],
      'pragma' => [Object],
      'content-type' => [Object],
      'expires' => [Object],
      'vary' => [Object],
      'content-encoding' => [Object],
      'content-length' => [Object],
      'keep-alive' => [Object],
      'connection' => [Object]
    },
    [Symbol(headers map sorted)]: null
  }
}