Magento 2: set default data for an ui component field from DataProvider

Question

I want to set a default value for an uiComponent field (shipping_id in this case) from the DataProvider (not from the form's XML, but from PHP). This is because the default value is dynamic (i am creating a new record based on a shipping id).

In my DataProvider:

    public function getData()
    {

        foreach ($this->collection->getItems() as $item) {
            $this->_loadedData[$item->getId()]['general'] = $item->getData();
        }

        if (empty($this->_loadedData)) {
            //set default values

            /** @var ShipmentInterface $shipment */
            $shipment = $this->persistor->get('shipment');
            $this->_loadedData['general']['shipment_id'] = $shipment->getEntityId();
        }

        return $this->_loadedData;
    }

and in my form's XML i am having a fieldset:

    <fieldset name="general">
        <argument name="data" xsi:type="array">
            <item name="config" xsi:type="array">
                <item name="label" xsi:type="string" translate="true">General</item>
            </item>
        </argument>
        <field name="shipment_id">
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                    <item name="dataType" xsi:type="string">number</item>
                    <item name="label" xsi:type="string" translate="true">Shipment ID</item>
                    <item name="formElement" xsi:type="string">hidden</item>
                </item>
            </argument>
        </field>
        ...
    </fieldset>

Anyway, the code above from the DataProvider is working partially, it loads existing data, but when adding, the hidden field has no value. Any idea how to reference a field from a fieldset when adding/editing data?

What should be the structure of $this->_loadedData when adding/editing data?