Hello @parthp,
Unfortunately, you are correct, and the only option to add a custom signup link using the New Universal Login is updating a Login template (e.g. placing the custom Signup link in the footer and disabling the registration at the connection level via Auth0 Dashboard), which requires a custom domain.
The custom Domains feature is available on all paid plans. Under some circumstances, we can sometimes enable the Custom Domains feature while you are on your Trial period.
Another option is switching to Classic Universal Login, which allows configuring the Custom Signup Link even without the Custom Domain. This might not suit you, as Organizations are unsupported in the Classic Experience.
Lock Configuration Options > signUpLink
Thanks,
Art