The payment code is shown on documents that Revenu Québec sends you (such as a notice of assessment or form) and on the electronic notice of assessment available in Revenu Québec's My Account for individuals. 

If you don’t have a payment code, you can also use your social insurance number (SIN).

If you don't have a payment code and you are not able to use your SIN to pay your Quebec taxes, you should contact Revenu Québec at 1 800 267-6299.