Adding tawk.to code just before body tag on functions.php file

You can the code to head or before body tag this way. Add this code at the last of functions.php

add_action( 'wp_head', function() {
    ?>
    <!--Start of Tawk.to Script-->
    <script type="text/javascript">
    var Tawk_API=Tawk_API||{}, Tawk_LoadStart=new Date(); (function(){ var s1=document.createElement("script"),s0=document.getElementsByTagName("script")[0]; s1.async=true; s1.src="https://embed.tawk.to/5b54359ee21878736ba23110/default"; s1.charset="UTF-8"; s1.setAttribute('crossorigin','*'); s0.parentNode.insertBefore(s1,s0); })();
    </script>
    <?php
} );

Hope it helps