Magento 2 : Add custom array in multiselect UI dropdown in custom UI form

You can add below code for to add in your UI XML file:

 <field name="category_id">
        <argument name="data" xsi:type="array">
            <item name="options" xsi:type="object">Namespace\VendorName\Model\Source\CategoryTree</item>
            <item name="config" xsi:type="array">
                <item name="label" xsi:type="string" translate="true">Category</item>
                <item name="componentType" xsi:type="string">field</item>
                <item name="formElement" xsi:type="string">multiselect</item>
                <item name="component" xsi:type="string">Magento_Ui/js/form/element/ui-select</item>
                <item name="elementTmpl" xsi:type="string">ui/grid/filters/elements/ui-select</item>
                <item name="dataScope" xsi:type="string">category_id</item>
                <item name="filterOptions" xsi:type="boolean">true</item>
                <item name="showCheckbox" xsi:type="boolean">true</item>
                <item name="disableLabel" xsi:type="boolean">true</item>
                <item name="sortOrder" xsi:type="number">60</item>
                <item name="multiple" xsi:type="boolean">true</item>
                <item name="levelsVisibility" xsi:type="number">0</item>
                <item name="required" xsi:type="boolean">true</item>
                <item name="source" xsi:type="string">sourceName</item>
                <item name="validation" xsi:type="array">
                    <item name="required-entry" xsi:type="boolean">false</item>
                </item>
                <item name="listens" xsi:type="array">
                    <item name="${ $.namespace }.${ $.namespace }:responseData" xsi:type="string">setParsed</item>
                </item>
            </item>
        </argument>
    </field>

Then add below code of Source Model of Category Tree:

<?php

namespace namespace\vendorName\Model\Source;

use Magento\Framework\Data\OptionSourceInterface;
use Magento\Catalog\Model\ResourceModel\Category\CollectionFactory as CategoryCollectionFactory;
use Magento\Framework\App\RequestInterface;
use Magento\Catalog\Model\Category as CategoryModel;

/**
 * Options tree for "Categories" field
 */
class CategoryTree implements OptionSourceInterface
{
    /**
     * @var \Magento\Catalog\Model\ResourceModel\Category\CollectionFactory
     */
    protected $categoryCollectionFactory;

    /**
     * @var RequestInterface
     */
    protected $request;

    /**
     * @var array
     */
    protected $categoriesTree;

    /**
     * @param CategoryCollectionFactory $categoryCollectionFactory
     * @param RequestInterface $request
     */
    public function __construct(
        CategoryCollectionFactory $categoryCollectionFactory,
        RequestInterface $request
    ) {
        $this->categoryCollectionFactory = $categoryCollectionFactory;
        $this->request = $request;
    }

    /**
     * {@inheritdoc}
     */
    public function toOptionArray()
    {
        return $this->getCategoriesTree();
    }

    /**
     * Retrieve categories tree
     *
     * @return array
     */
    protected function getCategoriesTree()
    {
        if ($this->categoriesTree === null) {
            $storeId = $this->request->getParam('store');
            /* @var $matchingNamesCollection \Magento\Catalog\Model\ResourceModel\Category\Collection */
            $matchingNamesCollection = $this->categoryCollectionFactory->create();

            $matchingNamesCollection->addAttributeToSelect('path')
                ->addAttributeToFilter('entity_id', ['neq' => CategoryModel::TREE_ROOT_ID])
                ->setStoreId($storeId);

            $shownCategoriesIds = [];

            /** @var \Magento\Catalog\Model\Category $category */
            foreach ($matchingNamesCollection as $category) {
                foreach (explode('/', $category->getPath()) as $parentId) {
                    $shownCategoriesIds[$parentId] = 1;
                }
            }

            /* @var $collection \Magento\Catalog\Model\ResourceModel\Category\Collection */
            $collection = $this->categoryCollectionFactory->create();

            $collection->addAttributeToFilter('entity_id', ['in' => array_keys($shownCategoriesIds)])
                ->addAttributeToSelect(['name', 'is_active', 'parent_id'])
                ->setStoreId($storeId);

            $categoryById = [
                CategoryModel::TREE_ROOT_ID => [
                    'value' => CategoryModel::TREE_ROOT_ID
                ],
            ];
            foreach ($collection as $category) {
                if ( $category->getData('level') <= 2 ) {
                    foreach ([$category->getId(), $category->getParentId()] as $categoryId) {
                        if ( !isset($categoryById[$categoryId]) ) {
                            $categoryById[$categoryId] = ['value' => $categoryId];
                        }
                    }
                    $categoryById[$category->getId()]['is_active'] = $category->getIsActive();
                    $categoryById[$category->getId()]['label'] = $category->getName();
                    $categoryById[$category->getParentId()]['optgroup'][] = &$categoryById[$category->getId()];
                }
            }
            $this->categoriesTree = $categoryById[CategoryModel::TREE_ROOT_ID]['optgroup'];
        }

        return $this->categoriesTree;
    }
}