https://github.com/lynndylanhurley/devise_token_auth/issues/456

registrations_controller.rb

class RegistrationsController < Devise::RegistrationsController

  def create
    super do |resource|
      if resource.persisted?
        resource.send_confirmation_instructions
      end
    end
  end
end

routes.rb

Rails.application.routes.draw do
  devise_for :users, controllers: { registrations: "registrations" }

  root 'application#home'
end