Ext JS: How do I dynamically fill a textfield with AJAX?

Shop.combo.Price = function(config) {
    config = config || {};
    Ext.applyIf(config,{
        allowBlank: true,
        anchor: '100%',
        baseParams: {
            action: 'mgr/products/getprice',
            resource: MODx.request['id']
        },
        description: 'Price',
        displayField: 'pagetitle',
        editable: false,
        enableKeyEvents: true,
        fieldLabel: 'Price',
        hiddenName: 'price',
        id: 'qsproduct-price',
        maxLength: 255,
        name: 'price',
        url: Shop.config.connector_url,
        value: 0,
        valueField: 'id',
        width: '100%'
    });
    Shop.combo.Price.superclass.constructor.call(this,config);
};
Ext.extend(Shop.combo.Price, MODx.combo.ComboBox,{
    _onblur: function(t,e) {
        var v = this.getRawValue();
        this.setRawValue(v);
        this.setValue(v,true);
    }
});
Ext.reg('Shop-combo-price', Shop.combo.Price);