magento-test/lib/web/mage/multiselect.html

<div class="admin__action-multiselect-search-wrap">
    <input class="admin__control-text admin__action-multiselect-search" data-role="advanced-select-text" type="text">
    <label class="admin__action-multiselect-search-label" data-action="advanced-select-search"></label>
    <div class="admin__action-multiselect-search-count">
        <span class="admin__action-multiselect-items-selected">0</span> selected
    </div>
</div>