Copy the proper database description file from "<JBoss-dir>/docs/examples/jca" to the deployment directory of your configured JBoss profile (for example "<JBoss-dir>/server/default/deploy") and customize it.

Restart JBoss

After that, the running JNDI 'nodes' can be checked using the JMX-Console (http://localhost:<port>/jmx-console), using the "listXML" operation. In my case, I had an entry saying

--- listXML results ---
java:

DB2DS
org.jboss.resource.adapter.jdbc.WrapperDataSource
--- /listXML results ---

From this I could easily derive the correct JNDI-name, which I configured as:

--- dbforms-config.xml ---
<dbconnection
    id = "1"
    isJndi = "true"
    name = "java:DB2DS"
    default = "true"
    username = "user"
    password = "password"
    connectionProviderClass =
        "org.jboss.resource.connectionmanager.LocalTxConnectionManager"
    conClass  = "COM.ibm.db2.jdbc.app.DB2Driver"/>
-- /dbforms-config.xml ---