Как очищать поле при выборе чекбокса в extjs?

{
                columnWidth: .5,
                layout: 'form',
                defaults: {msgTarget: 'under'},
                border: false,
                items: [{
                    xtype: 'hidden',
                    name: 'id',
                    id: config.id + '-id',
                }, {
                    xtype: 'textfield',
                    fieldLabel: _('extras_key_name'),
                    name: 'name',
                    id: config.id + '-name',
                    cls: 'apigenerator',
                    anchor: '99%',
                    allowBlank: true,
                }, {
                    xtype: 'textfield',
                    fieldLabel: _('extras_key_host'),
                    name: 'host',
                    id: config.id + '-host',
                    anchor: '99%',
                    allowBlank: true,
                }, {
                    xtype: 'extras-combo-user',
                    fieldLabel: _('extras_key_createdby'),
                    anchor: '99%',
                    allowBlank: true,
                }, {
                    layout: 'column',
                    border: false,
                    anchor: '100%',
                    items: [{
                        columnWidth: .50,
                        layout: 'form',
                        defaults: {msgTarget: 'under'},
                        border: false,
                        items: [{
                            xtype: 'xcheckbox',
                            boxLabel: _('extras_key_vip'),
                            name: 'vip',
                            id: config.id + '-vip',
                            checked: false,
                        }],
                    }, {
                        columnWidth: .50,
                        layout: 'form',
                        defaults: {msgTarget: 'under'},
                        border: false,
                        items: [{
                            xtype: 'xcheckbox',
                            boxLabel: _('extras_key_unlimited'),
                            name: 'unlimited',
                            id: config.id + '-unlimited',
                            checked: false,
                        }],
                    }],
                }]
            }