This is why I'm so glad the GDPR exists. They list their email address in the privacy policy so a quick email should suffice. Obviously you can raise complaints if they do not delete your information as the GDPR does not mandate or expect the individual to have to agree to another parties privacy policy to exercise your rights. You also have rights to access et al and whilst unlikely it's possible failure to act with…
The problem with the GDPR is that there is no enforcement. At best, you're relying on the other party acting in good faith and wanting to respect the regulation. I've had companies that breached the regulation, complained to them with no effect, raised it with the ICO, they got back to me, upheld my complaint, reached out to the company and still nothing despite them being based in the UK thus enforcing fines is triv…
https://www.enforcementtracker.com/
I'm not sure about how efficient the process is, but there is some enforcement.