I assume when you say that 'a new user is created' that you mean you are 'manually' creating this user either through Azure AD or O365 Admin?
It would add a bit more to what you're trying to do, but why don't you automate the user creation part, based on a SharePoint form, for instance, that someone fills out submits. From the Power Automate perspective, you can then trigger emails to go out once the card 'Create User in AzureAD' has finished creating the user account. I've done this whole onboarding process for licensed and guest users for a project I'm working on, so happy to provide some guidance and help.
I don't think there is a way to do the trigger you are asking for in your post above.