In OpenERP first you have to create the product and price of the product and then relate both price and product to the event form and return the value of the product and validate it to database like as in param self and param cr. Param self means that the object pointer and param cr means the current row of the database cursor and param uid means the current user’s ID for security checks.

Use this function show in given below

def onchangevent(self, cr, uid, ids, event_id, partner_invoice_id):
    
        context = {}
        if not event_id:
            return {'value': {'unit_price': False, 'event_product': False}}

        event_obj = self.pool.get('event.event')
        prod_obj = self.pool.get('product.product')
        res_obj = self.pool.get('res.partner')

        data_event =  event_obj.browse(cr, uid, event_id)
        res = {'value': {'unit_price': False,
                         'event_product': False,
                         'user_id': False,
                         'date': data_event.date_begin,
                         'date_deadline': data_event.date_end,
                         'description': data_event.note,
                         'name': data_event.name,
                         'section_id': data_event.section_id and data_event.section_id.id or False,
                        }}
        if data_event.user_id.id:
            res['value'].update({'user_id': data_event.user_id.id})
        if data_event.product_id:
            pricelist_id = data_event.pricelist_id and data_event.pricelist_id.id or False
            if partner_invoice_id:
                partner = res_obj.browse(cr, uid, partner_invoice_id, context=context)
                pricelist_id = pricelist_id or partner.property_product_pricelist.id
            unit_price = prod_obj._product_price(cr, uid, [data_event.product_id.id], False, False, {'pricelist': pricelist_id})[data_event.product_id.id]
            if not unit_price:
                unit_price = data_event.unit_price
            res['value'].update({'unit_price': unit_price, 'event_product': data_event.product_id.name})
        return res