Code :

# -*- coding: utf-8 -*-
from odoo import models

class EventEvent(models.Model):
_inherit = 'event.event'

@api.model_create_multi
def create(self, vals_list):
"""
Create registration records and automatically generate calendar events
if both start and end dates are provided.
"""
res = super().create(vals_list)
for registration in res:
if registration.date_begin and registration.date_end:
self.env['calendar.event'].create({
'name': registration.name,
'start': registration.date_begin,
'stop': registration.date_end,
'allday': False,
'description': registration.description,
'user_id': registration.user_id.id if registration.user_id else False,
'attendee_ids': [(6, 0, registration.user_id.id)],
})
return res