According to the Google help site there are four available methods for adding Google site verification to your site. (Please see this article: https://support.google.com/webmasters/answer/35179?hl=en
) To do this you would need to use the following method:
Add the Google Analytics code you use to track your site. To use this option, you must be an administrator on the Google Analytics account, and the tracking code must use the new asynchronous snippet.
To do this in your Pressero site, you would add the code in the Analytics script area found in Sites > [site name] > Settings > SEO/HTML tab.