I'd also like to know this. It seems like asking this organisation to delete my data would be largely beneficial, but what data do I need to provide for them to do it?
According to GDPR, the contact info for sending an access or deletion request must be provided in the Privacy Policy.
Under GDPR (Europe), if you send a request, the company must honor it unless they have reason to doubt your identity, in which case they must ask for follow-up. Under CCPA (California), they are only obligated to honor "verified" requests. There's a range of what counts as verifying, from just being able to log in to your account on the low end, up to providing 3 pieces of matching data on the high end.
The company is obligated to tell you what data they have. They are not obligated to go out of their way to make connections, though, so you're better served by providing as many identifiers as possible (like account numbers).
