You may add hashtags from the caption or remarks of your article. If you add hashtags into a post that is set to people, the article will be visible on the corresponding hashtag webpage.

To label a photo or video using a hashtag:

Upload or take a photo or movie
Choose to add a filter, then type # followed by text or emoji from the Caption area (ex: #flower)
If You Wish to add a hashtag to a post you have already uploaded, edit the caption or add your hashtag in a comment in your photo
Once you tag your post with a hashtag, you will have the ability to tap the hashtag into seeing a webpage that shows all videos and photos people have uploaded with that hashtag.

How to use Instagram hashtags - Instajool.com Blog

Things to Bear in Mind:

When individuals with personal profiles label articles, they won’t appear publicly on hashtag pages.
Amounts are permitted in hashtags. However, spaces and special characters, like $ or percent, will not work.
You can just label your own posts. You can not tag other people’s photos/videos. If you include more than 30 tags on one photo/video, your comment will not post.