Hi Tomoko,
As you are already having "Tracking Code" , I assume you are using "Google Analytics Tracking Code" as one of the method to verify the site owner ship . your code must be similar to the below tracking code :
<script>
(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','https://www.google-analytics.com/analytics.js','ga');
ga('create', 'UA-49XXXXXX-1', 'auto');
ga('send', 'pageview');
</script>
So in this case , you can Add this code via DTM by "Adding a Google Universal Analytics Tool " to you property and use "UA-49XXXXXX-1" as the Production and Staging Account ID and use the Tracker name as "GA Default" and then use setting that Google Analytics code is hosted through "URL" : https://www.google-analytics.com/analytics.js . Please see the following link for more info :
https://marketing.adobe.com/resources/help/en_US/dtm/google_universal_analytics.html
Thanks & Regards
Parit Mittal