Can't add users to crowd with rest API, getting Internal Server Error 500

[Basel]

import.java.net.HttpURLConnection
import java.net.URL
import java.util.Base64

URL url = new URL(http://crowd:8095/crowd/rest/usermanagement/1/user
con.setRequestMethod("POST")

def password = "password"
def username = "username"
def userpass = username + ":" + password
def basicAuth = "Basic:" + new String(Base64.getEncoder().encode(userpass.getBytes())

con.setRequestProperty("Authorization", basicAuth)
con.setRequestProperty(Content-Type", application/json; utf-8)
con.setRequestProperty("Accept", aplication/json)
con.setRequestProperty("Cache-Control","no-cache")
con.setRequestProperty("X-Atlassian-Token", "no-check")

con.SetDoOutput(true)
def body = "{\"name\": \"sampleuser\",\"password\":\"secret\",\"first-name\":\"sample\",\"last-name\":\"user\",\"display-name\":\"Sample User\",\"email\":\"sampleuser@user.cool\"}"

OutputStream os = con.getOutputStream()
byte[] input = body.getBytes("utf-8")
os.write(input,0,input.length)

log.warn(con.getResponseCode)
log.warn(con.getResponseMessage)
con.disconnect()

I've written the script as above, which should add a user crowd. However, when it runs I get response code 500 "Internal Server Error".

Any help would be appreciated thanks :) 

[Serj Shcherbakov]

Hi Thanos,
Your solution does not works: now assignee field does not populate on creation form,
PFA:

[Thanos Batagiannis [Adaptavist]]

Hi Сергей,

The id of the assignee field is wrong, and since you already import the IssueFieldConstants (good practice) then 

def assignee = getFieldById(ASSIGNEE)

Regards, Thanos