<referenceContainer name="content">
   <container name="button.actions.container" label="Container">
     all button action here .. such as save, back reset
   </container>
   <container name="form" label="Form"><!-- this modal form -->
    <block class="NamespaceModuleBlockAdminhtmlFormWrapForm" >
       <uiComponent name="ui_form"/>
     </block>
   </container>
</referenceContainer>

$meta['add_item_modal']['children']['create_item_form_modal'] = [
        'arguments' => [
            'data' => [
                'config' => [
                    'isTemplate' => false,
                    'componentType' => ComponentContainer::NAME,
                    'dataScope' => 'data.item_form',
                    'provider' => 'item_form.megamenu_item_add_form_data_source',
                    'options' => [
                        'title' => __('Header Title below')
                    ]
                ]
            ]
        ],
        'children' => [
            static::FORM_MODAL_FIELDSET => [
                'arguments' => [
                    'data' => [
                        'config' => [
                            'label' => __('Fieldset'),
                            'componentType' => Fieldset::NAME,
                            'dataScope' => 'item',
                            'collapsible' => true,
                            'sortOrder' => 10,
                            'opened' => true,
                        ],
                    ],
                ],
                'children' => [
                    static::FIELD_NAME_1 => $this->getFirstFieldConfig(10),
                    static::FIELD_NAME_2 => $this->getSecondFieldConfig(20)
                ],
            ],
            static::ITEM_FORM => $this->getFormConfig() // <== UI FORM

        ]
    ];

protected function getFormConfig()
{
    return [
        'arguments' => [
            'data' => [
                'config' => [
                    'label' => __('Form'),
                    'componentType' => ComponentContainer::NAME,
                    'component' => 'Vnecoms_Megamenu/js/form/components/item-insert-form',
                    'dataScope' => '',
                    'ns' => static::ITEM_FORM,
                    'update_url' => $this->urlBuilder->getUrl('mui/index/render'),
                    'render_url' => $this->urlBuilder->getUrl(
                        'mui/index/render_handle',
                        [
                            'handle' => 'megamenuadmin_item_form',
                            'buttons' => 1
                        ]
                    ),
                    'autoRender' => false,
                    'externalProvider' => static::ITEM_FORM
                        . '.megamenu_item_add_form_data_source',
                    'toolbarContainer' => '${ $.parentName }',
                    'formSubmitType' => 'ajax',
                    'imports' => [
                    ],
                    'exports' => [
                    ]
                ]
            ]
        ]
    ];
}

<page>
    <referenceContainer name="content">
     <uiComponent name="megamenu_item_add_form"/>
   </referenceContainer>
 </page>

define([
   'Magento_Ui/js/form/components/insert-form'
], function (InsertForm) {
'use strict';

return InsertForm.extend({
    defaults: {
        modules: {
            itemForm: 'item_form.item_form'
        },
        listens: {

        },
        itemId: 0
    }

    });
});