Use this link
https://www.canada.ca/en/immigration-refugees-citizenship/corporate/contact-ircc/web-form.html
"Update or ask about your application"
"Add document to your application"
Fill out the form and were it asks for your application number, click on the "By checking this box, I confirm that I don’t know my application number" box.
Use your UCI instead (there's a box further in the form to provide it).
In the last section where you can write your comments, explain that you forgot to pay the biometric fees and that you're attaching the receipt for biometrics.
I did the same thing. I applied without paying biometric fees, read in this forum that I should have, paid and sent it through webform a few weeks after applying. Got BIL along with AOR.