How to include tag in the post url?

I have an instance of Ghost running on DO. Currently, all my post URLs are like this mywebiste.com/post-slug. I want to change it to mywebsite.com/tag/post-slug.

How do I do it?

This would be a use case for custom routing.

In particular, you can update your main collection to:

routes:

collections:
  /:
    permalink: /{primary_tag}/{slug}/
    template: index

taxonomies:
  tag: /tag/{slug}/
  author: /author/{slug}/

Now, a post with the tag special and slug title would appear on /special/title.

Thanks, @RyanF this solved my problem. If I want my posts to normally appear as site.com/slug and also as site.com/my_tag/slug, how can I do that?

A post can only have one URL. You can set up a redirect so that one URL redirects to the other, but a post can’t live at both locations. You also likely don’t want to do that because you’d get penalized by search engines, which would see you as having duplicate content.