Welcome back to the Auth0 Community,
As stated in this Knowledge Solution → Send a Welcome Email on Request, it’s not possible to send a welcome email on demand. A possible workaround to this problem is to set up a third-party provider that you would add to your signup flow.
I would encourage you to create a feature request under Feedback category, here is our link: Feedback - Auth0 Community. You can read our FAQ regarding submitting a feature request here: How to Submit Product Feedback or Feature Requests
Thanks
Dawid