Inside {{#foreach posts}} block, you can test if a post has a specific tag with:
{{#has tag="tagname"}}..{{/has}}
If you want a custom template for a particular tag archive with an extra div, then you can duplicate tag.hbs and name it tag-[slug].hbs to use that template instead
I probably wrote incorrectly. I work with main tag page template (tag.hbs), and need to add additional div to this template. But I need to show this block only on specific tag’s page. In other words, I need to determine which tag page I am currently on, and if the desired tag is used, then show the div.