A request XML packet creating database user account for the database includes the add-db-user operation node:
<packet version="1.4.2.0">
<database>
<add-db-user>
...
</add-db-user>
</database>
</packet>
The add-db-user node is presented by type DatabaseAddDBUserInputType (database_input.xsd), and its graphical representation is as follows:

Remarks
You can add multiple users to database in a single packet. Add as many add-db-user operations to the packet as the number of different users you want to create. You can also add multiple users to multiple databases in a single packet.
<database>
<add-default-user>
...
</add-default-user>
...
<add-default-user>
...
</add-default-user>
</database>