https://www.reddit.com/settings/data-request
Having worked at a company that had a massive influx of GDPR requests we weren’t prepared for, this one could actually cause them some trouble if Reddit don’t have that process properly automated.
Also if you’re feeling extra cantankerous you could try emailing them directly: redditdatarequests@reddit.com
Even though they say you have to use their form, they can’t actually force you to do that: https://ico.org.uk/for-organisations/uk-gdpr-guidance-and-resources/individual-rights/right-of-access/how-do-we-recognise-a-subject-access-request-sar/#requirements