Creating Drupal Accounts when a sub-type CiviCRM Contact is created - created by a webform
Most of what you need is here: How can I create a Drupal user when someone submits a CiviCRM-Webform? based on @petednz-fuzion's blog post.   It uses civicrm_entity which has seen a lot of activity since the previous question so there may be some changes but hopefully nothing that prevents you following it.

For limiting action to a specific contact sub-type, add a new Condition using 'Data comparison' with a data selector of civicrm-contact:contact-sub-type