add a key listener to /!Admin/adminUIlogon.ui.js.

Ext.onReady(function(){
    var formLogin = new Ext.FormPanel({
        frame: false, border: false, buttonAlign: 'center',
        url: BASE_URL   'user/extjs_login', method: 'POST',
        id: 'frmLogin',
        items: [{
            xtype: 'textfield',
            fieldLabel: 'Username',
            id:'fldusername',
            name: 'username',
            allowBlank: false
        }, {
            xtype: 'textfield',
            fieldLabel: 'Password',
            id:'fldpassword',
            name: 'password',
            allowBlank: false,
            inputType: 'password'
        }],
        buttons: [
            { text: 'Login' },
            { text: 'Reset', handler: function() {
                    formLogin.getForm().reset();
                }
            }
        ],
        keys: [
            { key: [Ext.EventObject.ENTER]</font>, handler: function() {
                    Ext.Msg.alert("Alert","Enter Key Event !");
                }
            }
        ]
</span>    });

    var winLogin = new Ext.Window({
        title: 'Extjs Enter Key Event', layout: 'fit',
        width: 340, height: 140, resizable: false,
        closable: false, items: [formLogin]
    });

    winLogin.show();
});