Add HTML code before the title of the Tag page

First, in WordPress lingo ‘content’ does not include ‘title’. (Also, be careful with ‘title’. Sometimes it means ‘the heading above the main content, and below the site navigation and/or masthead’, and sometimes it means the html title, which is a piece of text the browser shows at the top of the window or tab, in its history, etc. In this case, it sounds like you’re referring to the former.

Second, in this case there isn’t a hook to modify the title. Instead, modify a template. You might worry that you’d have to modify several template files, since web pages for the site will look different. Fortunately, usually, for consistency the top part of different kinds of web pages will look the same. So, you probably just have to modify header.php or some template part (probably in template-parts/header).