Combobox hiddenName not supported?

[gubespam]

I am using the ExtJS 4.0 preview. When running inside Firebug, I get an exception "uncaught exception: hiddenName is no longer supported." What is the replacement for hiddenName in 4.0 preview, or else, how does one submit a model field with a form that is different from the model field used as the display field?

    var customTplCombo = new Ext.form.ComboBox({
        hiddenName: 'clientId',
        fieldLabel: 'Client Name',
        displayField: 'clientName',
        store: clientsStore,
        queryMode: 'local',
        forceSelection: true // force user to choose an option, not arbitrary text
    });