Custom ui_component form fieldset not submitting

I created a custom fieldset for a ui_component form to add custom input structures. The problem is, that the inputs are not included in the POST values.

The input you see is just a simple element in the fieldset.phtml you see in the Block code. It has a custom name “test_input_data”. I implemented the fieldset similar to the conditions fieldsets on sales rules. The form namespance is shipping_rule_form.

    
        true
        Conditions
    
    
        
            
        
    

In the Block i am adding the fieldset like this:

protected function _prepareForm()
{
    $model = $this->_coreRegistry->registry('current_shipping_rule');
    $form = $this->addTabToForm($model);
    $this->setForm($form);

    return parent::_prepareForm();
}

protected function addTabToForm($model, $fieldsetId = 'conditions_fieldset', $formName = 'shipping_rule_form')
{
    if (!$model) {
        $id = $this->getRequest()->getParam('entity_id');
        $model = $this->_ruleFactory->create();
        $model->load($id);
    }
    $conditionsFieldSetId = 'shipping_rule_form_conditions';

    /** @var MagentoFrameworkDataForm $form */
    $form = $this->_formFactory->create();
    $form->setHtmlIdPrefix('rule_');
    $renderer = $this->_rendererFieldset->setTemplate(
        'Remklov_ShippingRule::promo/fieldset.phtml'
    )->setFieldSetId(
        $conditionsFieldSetId
    );

    $fieldset = $form->addFieldset(
        $fieldsetId,
        [
            'legend' => __(
                'Apply the rule only to the products matched by the rule, if following conditions are met.'
            )
        ]
    )->setRule(
        $model
    )->setRenderer(
        $renderer
    );
    $fieldset->addField(
        'conditions',
        'text',
        [
            'name'           => 'conditions',
            'label'          => __('Conditions'),
            'title'          => __('Conditions'),
            'required'       => true,
            'data-form-part' => $formName
        ]
    );

    $form->setValues($model->getData());
    return $form;
}

When i do a print_r() on the getParams() in the Save-Controller, it just returns the data of the corresponding model, but not the custom input value. I also tried to add the conditions exactly like in the sales_rule_form.xml, but still nothing is posted.

I could not find out how the submit logic works here and where i maybe missed or need to add logic.