In Odoo11, I can get transaction reference by this way
transaction = request.website.sale_get_order().payment_tx_id
transaction.reference
But in Odoo13, payment_tx_id
field has removed from sales order,
Field Many2one (payment_tx_id) to Many2many (transaction_ids) type changed from Odoo12.
Try with following code:
order = request.website.sale_get_order()
#if you need to get last transaction
transaction = order.get_portal_last_transaction()
reference = transaction.reference
#if you need all reference to listed
reference = str(', '.join(order.transaction_ids.mapped('reference')))