Magento – magento2 add script tag right after open body tag

I want to add script tag (google tag manager) right after opening body-tag. I added a referenceContainer to default.xml – but content of "afterbody.phtml" is allways printed AFTER header.container instead of before.

Thanks for help!
martin

    <referenceContainer name="after.body.start">
        <block class="Magento\Framework\View\Element\Template" name="afterbody" before="header.container" template="Magento_Theme::afterbody.phtml"/>
    </referenceContainer>
    <referenceContainer name="header.container">
        <container name="header.panel" label="Page Header Panel" htmlTag="div" htmlClass="panel header">
            <block class="Magento\Framework\View\Element\Template" name="skip_to_content" template="Magento_Theme::html/skip.phtml">
                <arguments>
                    <argument name="target" xsi:type="string">contentarea</argument>
                    <argument name="label" translate="true" xsi:type="string">Skip to Content</argument>
                </arguments>
            </block>
            <block class="Magento\Store\Block\Switcher" name="store_language" as="store_language" template="switch/languages.phtml"/>
            <block class="Magento\Framework\View\Element\Html\Links" name="top.links">
                <arguments>
                    <argument name="css_class" xsi:type="string">links</argument>
                </arguments>
            </block>
        </container>
        <referenceBlock name="compare" remove="true" />
        <referenceBlock name="wish-list-link" remove="true" />
    </referenceContainer>
    <referenceContainer name="columns">
        <container name="div.sidebar.main" htmlTag="div" htmlClass="sidebar sidebar-main-1" before="main">
            <container name="sidebar.main" as="sidebar_main" label="Sidebar Main"/>
        </container>
        <container name="div.sidebar.additional" htmlTag="div" htmlClass="sidebar sidebar-additional1" after="div.sidebar.main">
            <container name="sidebar.additional" as="sidebar_additional" label="Sidebar Additional"/>
        </container>
    </referenceContainer>

    <referenceBlock name="top.links">
        <referenceBlock name="register-link" remove="true" />           <!--for Create Account Link-->
        <referenceBlock name="wish-list-link" remove="true" />          <!--for WishList Link-->
        <referenceBlock name="catalog.compare.link" remove="true" /> 
    </referenceBlock>