If you'd like to add hashtags to your post, you'll have to select your service, and add the symbol "#" in the compose box in order to start typing the hashtag you'd like to add.
Once you have started typing this, results will appear in a drop-down list beneath it in order for you to select the specific hashtag you'd like to add.
This feature is currently only available for Twitter and Instagram. If you would like to create a post which includes both of these services and includes one or more hashtags for each service, when typing a hashtag in the main tab, you'll find that the default list which appears displays the available hashtags for Twitter.
In order to add hashtags for your Instagram service you'll have to switch over to the Instagram tab and add the hashtag there as well.