<pre>

<fieldset name="display_settings">
        <field name="custom_product_list_template">
            <argument name="data" xsi:type="array">
                <item name="options" xsi:type="object">                NamespaceCategoryCustomModelCategoryAttributeSourceTemplates
                </item>
                <item name="config" xsi:type="array">
                    <item name="sortOrder" xsi:type="number">210</item>
                    <item name="dataType" xsi:type="string">string</item>
                    <item name="formElement" xsi:type="string">select</item>
                    <item name="label" translate="true" xsi:type="string">Ground Custom Template</item>
                </item>
            </argument>
        </field>
    </fieldset>

`<event name="layout_load_before">
        <observer name="add_product_list_template" instance="NamespaceCategoryCustomObserverProcessProduclistTemplate"/>
</event>`

<pre>public function execute(MagentoFrameworkEventObserver $observer)
{
    if($observer->getFullActionName() == 'catalog_category_view'){
        $currentCategory = $this->_coreRegistry->registry('current_category');
        if(is_object($currentCategory) && $currentCategory->getId()){
            $handle = $currentCategory->getData('custom_product_list_template') ?: '';
            if($handle){
                $observer->getLayout()->getUpdate()->addHandle($handle);
            }
        }
    }
    return $this;
}</pre>