From Twitter Help:
"Hashtags are a community-driven convention for adding additional context and metadata to your tweets. They're like tags on Flickr, only added inline to your post. You create a hashtag simply by prefixing a word with a hash symbol: #hashtag." To read more, click here.
Please select the option "Add Hashtag" from the Options input box
In the "Add Hashtag" input box, please input only the hashtag. No need for "#" symbol