Tags can only be applied to a comic in your collection. So if you're looking to add a tag, make sure it is already within your collection.
When using the My Details modal (the three dot menu), you can add a tag in the text field of the first tab. Enter the tag you'd like and click the "Add" button.
When on any comic page, you can add a tag through the My Details tab. From here, enter the tag at the top and click the "Add" button.