Register new account via REST application
Since that's straight drupal I would try the drupal docs and if you're still stuck you might get more responses at https://drupal.stackexchange.com.
Another thing you could try but I don't know if it will work is to use civi's api Profile.submit. If you have a profile at admin - customize - profiles which is configured for Drupal User Registration and allows creating users, you might be able to pass in the right parameters to include the account name/email to create the account.