Hi @Harj34 one of the best benefits of using your own domain name for your landing page is subscribers/viewers can see the branding within the URL. It may look more professional to others if you use your own domain name. But if you have your own website active on your domain let say for example your website is working and active with yourdomain.com30, if you wish to still use your domain but not affect the current website you can use a Subdomain like newsletter.yourdomain.com27 or landingpage.yourdomain.com22.
You can also check this article here: Account domains vs. custom domains84
I hope it helps. If you're still having issues you can always reach out to firstname.lastname@example.org.