extjs: custom tpl for combobox list option

extjscombobox.js

{
        xtype: 'modx-combo'
        ,width: 200
        ,name: "id"
        ,id: config.id + '-user-field'
        ,url: MODx.config.connectorUrl
        ,fields: ['id', 'firstname', 'lastname']
        ,tpl: '<tpl for="."><div class="x-combo-list-item" >{firstname} {lastname}</div></tpl>'
        ,displayField: 'firstname'
        ,baseParams: {
            action: 'mgr/user/getlist'
            ,limit: 0
        }
        ,listeners: {
            'select': {
                fn:this.filter,scope: this
            }
        }
    }