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{{block class="Magento\Cms\Block\Block" block_id="your_block_identifier"}}

<block class="Magento\Cms\Block\Block" name="header_menu">
    <arguments>
        <argument name="block_id" xsi:type="string">header_menu</argument>
    </arguments>
</block>

<?php echo $block->getLayout()->createBlock('Magento\Cms\Block\Block')->setBlockId('block_identifier')->toHtml();?>