Fancy MODX / ExtJS combo-box that searches while typing and adds new entries if they don't exist.

1. In the ExtJS form declaration.js

// Call the form with the unique xtype
items: [{
  xtype: 'campermgmt-newcamper-form-brandscombo',
  fieldLabel: 'Merknaam',
  name: 'brand',
  id: 'brand',
  allowBlank: false,
  vtype: 'alphanum'
}
<...>

// Declare the actual xtype
CamperMgmt.formGeneral.BrandsCombo = function(config) {
    config = config || {};
    Ext.applyIf(config,{
        url: CamperMgmt.config.connectorUrl,
        baseParams: {
            action: 'mgr/index/getbrands'
        },
        fields: ['id','name'],
// Editabel makes you able of typing in the combobox
        editable: true,
// typeAhead sends posts for every character you type in (starting at minChars) to search for existing matches.
        typeAhead: true,
        minChars: 1,
// disabling forceSelection allows you to use a non-existent value
        forceSelection: false
    });
    CamperMgmt.formGeneral.BrandsCombo.superclass.constructor.call(this,config);
    };
Ext.extend(CamperMgmt.formGeneral.BrandsCombo,MODx.combo.ComboBox);
Ext.reg('campermgmt-newcamper-form-brandscombo',CamperMgmt.formGeneral.BrandsCombo);

save processor.php

// This file is posted to when the entire form gets submitted, and does the magic of adding the field if it doesn't exist.

// Find the object based on the "brand" posted value
$brandObj = $modx->getObject('cmBrand',array('name' => $scriptProperties['brand']));
// If it is not empty (ie it exists), simply add it to $c, which is the camper record
if (!empty($brandObj)) {
    $c->addOne($brandObj);
}
// If it doesn't exist, create it and add it to the camper record.
else {
    $brandObj = $modx->newObject('cmBrand');
    $brandObj->set('name',$scriptProperties['brand']);
    $brandObj->save();
    $c->addOne($brandObj);
}

the getbrands processor.php

// This file actually searches for the search term, and is being posted to on every character typed in
// Get the search query. Note that I put it in $search cause I already used $query as the xPDOQuery object
$search = $modx->getOption('query',$scriptProperties,'');

$query = $modx->newQuery('cmBrand');
$query->sortby($sort,$dir);

// If there's a search request, tell modx to find it
if ($search !== '') {
    $query->where(array(
        'name:LIKE' => '%'.$search.'%'
    ));
}

//Output results as JSON