That's not possible on Discord. They require a phone number. Many countries have Mandatory SIM Registration laws that make it illegal to get a phone number anonymously.
And Discord will auto-ban your account just for using Tor Browser or other privacy addons in your web browser.
My point is: don't use Discord. Use a service that allows you to only use a throwaway email.
Message sent from Tor Browser using an account on Lemmy registered with a throwaway email