newsupernova/vendor/magento/module-newsletter/view/adminhtml/templates/preview/store.phtml

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

// @codingStandardsIgnoreFile

?>

<?php if ($websites = $block->getWebsites()): ?>
<div class="field field-store-switcher">
    <label class="label" for="store_switcher"><?= $block->escapeHtml(__('Choose Store View:')) ?></label>
    <div class="control">
        <select
            id="store_switcher"
            class="admin__control-select"
            name="store_switcher">
            <?php foreach ($websites as $website): ?>
                <?php $showWebsite = false; ?>
                <?php foreach ($website->getGroups() as $group): ?>
                    <?php $showGroup = false; ?>
                    <?php foreach ($block->getStores($group) as $store): ?>
                        <?php if ($showWebsite == false): ?>
                            <?php $showWebsite = true; ?>
                            <optgroup label="<?= $block->escapeHtmlAttr($website->getName()) ?>"></optgroup>
                        <?php endif; ?>
                        <?php if ($showGroup == false): ?>
                            <?php $showGroup = true; ?>
                            <optgroup label="&nbsp;&nbsp;&nbsp;<?= $block->escapeHtmlAttr($group->getName()) ?>">
                        <?php endif; ?>
                        <option value="<?= $block->escapeHtmlAttr($store->getId()) ?>"<?php if ($block->getStoreId() == $store->getId()): ?> selected="selected"<?php endif; ?>>&nbsp;&nbsp;&nbsp;&nbsp;<?= $block->escapeHtml($store->getName()) ?></option>
                    <?php endforeach; ?>
                    <?php if ($showGroup): ?>
                        </optgroup>
                    <?php endif; ?>
                <?php endforeach; ?>
            <?php endforeach; ?>
        </select>
    </div>
    <?= $block->getHintHtml() ?>
</div>
<script>
require(['prototype'], function(){

//<![CDATA[
Event.observe($('store_switcher'), 'change', function(event) {
    var element = Event.element(event);
    $('preview_store_id').value = element.value;
});
//]]>

});
</script>
<?php endif; ?>