Answer: 

The module Magento 2 B2B Registration does not support setting the required field for Tax ID on the B2b form.

However, the default Magento does allow showing Tax/Vat ID on the create account default form.

Please follow this screenshot to set up:


This is the result on the B2b form for your reference:


If you have any other questions, please feel free to contact us.

We'd like to help.