A request XML packet retrieving database administrator info includes the get-default-user operation node:
<packet version="1.4.2.0">
<database>
<get-default-user>
¡
</get-default-user>
</database>
</packet>
The get-default-user node is presented by type DatabaseGetDBInputType (database_input.xsd), and its graphical representation is as follows:

Remarks
You can retrieve ID's of multiple database administrators using a single packet. Add the get-default-user operation for each database to the request packet.
<database>
<get-default-user>
¡
</get-default-user>
...
<get-default-user>
¡
</get-default-user>
</database>