HarmonyOS POST form-data方式body如何组织？

export const objectToQueryString = (params: object) => { 
  let result = "" 
  for (let key in params) { 
    const value = params[key] 
    result += `${key}=${value}&` 
  } 
  result = result.slice(0, -1) 
  return result 
} 
// 封装POST请求 
export const POST = (url: string, params?: object, header?: object): Promise<string> => { 
  return http.createHttp() 
    .request(url, { 
      method: http.RequestMethod.POST, 
      extraData: objectToQueryString(params), 
      header: { 
        ...(header || {}), 
        'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8', 
      }, 
    }) 
    .then(res => { 
      return res.result.toString() 
    }) 
} 
// 实际调用 
url = '访问地址' 
data = { username: 'CHJOO', password: 'CHJOO Is me' } //传递参数 
POST(url + "/user_login/", { username: this.LoginStudentId, password: this.LoginPassWord }).then(res) => 
{ 
  console.info("res is " + res) 
}