<?php
 
$service = $this->app->make('Concrete\Core\Attribute\Category\CategoryService');
$category = $service->getByHandle('collection');
 
$key = $category->getByHandle('stack_row');
 
if (!is_object($key)) {
    $pkg = Package::getByHandle('your_package_handle');
    $selectAttributeType = \Concrete\Core\Attribute\Type::getByHandle('select');
 
    //the options you want to add to your select attribute
    $optionsList = ['option 1', 'option 2'];
 
    $selectSettings = new \Concrete\Core\Entity\Attribute\Key\Settings\SelectSettings();
    $list = new \Concrete\Core\Entity\Attribute\Value\Value\SelectValueOptionList();
 
    $options = [];
    $displayOrder = 0;
    foreach ($optionsList as $option) {
        $opt = new \Concrete\Core\Entity\Attribute\Value\Value\SelectValueOption();
        $opt->setSelectAttributeOptionValue($option);
        $opt->setDisplayOrder($displayOrder);
        $opt->setOptionList($list);
        if (is_object($opt)) {
            $options[] = $opt;
            ++$displayOrder;
        }
    }
 
    $list->setOptions($options);
    $selectSettings->setOptionList($list);
 
    // set the other options for your attribute
    $selectSettings->setAllowOtherValues(true);
    $selectSettings->setHideNoneOption(true);
 
    $key = new CollectionAttributeKey();
    $key->setAttributeKeyHandle('stack_row');
    $key->setAttributeKeyName('Stack Row');
    $key = $category->add($selectAttributeType, $key, $selectSettings, $pkg);
}