I think you want to use the Rest API to create the users rather than Tabcmd. - Reference-Tableau Server REST API
Check that out and see if that doesn't better fit your needs here.
According to this:
Required Columns in the CSV File
The following values are required for each user:
- User name
- Password: If Tableau Server is configured to use Active Directory authentication, there must be a
Passwordcolumn, but the column itself should be empty. If the server is using local authentication, you must provide passwords for new users.
Hope this helps,