You can upload an web widget in the same manner as uploading a desktop widget. Click on the 'Upload Widget' button on the left of any Gallery page, fill in the widget details and the widget files for the desired online platforms. You need to be logged in to upload widgets. If you are not logged in, you can log in or create an account on the fly from the same upload popup.

After you upload your widget you will be able to set its parameters, using the 'Parameters' link, as described in more detail here.