Groovy POST request with form filling in it

I need to write a script similar to this cURL

curl -X 'POST' 'url/authenticate' -H 'accept: */*' -H 'Content-Type: multipart/form-data' -F 'Username=user_name' -F 'Password=pass_word'

I have this, but Request Response Code 400 says that Password field is required and Username field is required

import groovyx.net.http.HTTPBuilder
import groovyx.net.http.ContentType
import groovyx.net.http.Method

def http = new HTTPBuilder('url/authenticate')

def body = [
    "Username": 'user_name',
    "Password": 'pass_word'
]

http.request(Method.POST, ContentType.URLENC) {
    headers.'Accept' = '*/*'
    body = body
    response.success = { resp, reader ->
        log.warn("Response Status: ${resp.statusLine}")
        log.warn("Response Data: ${reader}")
    }
    response.failure = { resp, reader ->
        log.warn("Error Response Status: ${resp.statusLine}")
        log.warn("Error Response Data: ${reader}")
    }
}

How correctly pass form fields in this script?

P.S. POST request works fine from Postman, that provide this form fields in request body

public static String getAuthToken(){
        def http = new HTTPBuilder(url + "some/url")   
        http.request (POST) { multipartRequest ->
            MultipartEntityBuilder multipartRequestEntity = new MultipartEntityBuilder()
            String key = 
            multipartRequestEntity.addPart('Username', new StringBody(user))
            multipartRequestEntity.addPart('Password', new StringBody(password))

            multipartRequest.entity = multipartRequestEntity.build()

            response.success = { resp, data ->
                return data['token']
            }
            response.failure = { resp, data ->
                return data
            }
        }
    }

U can perform form filling by using MultipartRequestEntityBuilder