sending request to reCAPTCHA Enterprise API fails

I am trying to create ReCaptcha assessment using REST API in my backend server, using the following documentation: recaptcha enterprise rest referance

From documentation I understand that request body contains instance of Assesment, but when I try to send a request I receive the following error:

TypeError: Object of type Assessment is not JSON serializable

my code:

import requests
from google.cloud import recaptchaenterprise_v1
from google.cloud.recaptchaenterprise_v1 import Assessment

def create_assessment(project_id: str, recaptcha_site_key: str, token: str, recaptcha_action: str, apiKey:str):

    # Create event object
    event = recaptchaenterprise_v1.Event()
    event.site_key = recaptcha_site_key
    event.token = token

    # Create assesment object
    assessment = recaptchaenterprise_v1.Assessment()
    assessment.event = event
    
    # Set project name
    project_name = "projects/"+project_id

    response = requests.post(url="https://recaptchaenterprise.googleapis.com/v1/"+project_name+"/assessments?key="+apiKey, json=assessment)

    return response

I tried to convert Assesment to be json using dumps() but without success.

also tried to write it as "skinny json":

assessment = {'event':
                  {'token':token,
                  'siteKey':recaptcha_site_key,
                  'expectedAction':'LOGIN'}
              }

although I receive status code 200 it's indicate that my request is MALFORMED (probably because I don't include some recaptchaenterprise_v1 objects that should be on Assesment)