Unfortunately, Facebook does not currently allow services like Juicer to search for posts based on a hashtag. Therefore, it's not possible to add a Facebook source into a Juicer feed based on a hashtag.
If you have any other Facebook source listed in your "Social Media Sources" panel of Juicer, you do have the ability to manually add posts from other users. Click the source within that panel, select "Add post manually", and you will be able to paste in the URL of a Facebook post.
This technique, however, rarely works with posts made by personal Facebook accounts, due to privacy restrictions and other issues. Generally, this is not a reliable workaround for adding Facebook hashtag posts, and Juicer cannot guarantee you will be able to manually add Facebook posts into a Juicer feed.
If you do plan to manually add Facebook posts, please ensure you have connected a personal Facebook account on your Connected Social Accounts page, as this will provide the best chance of success.