You will need to have administrator privileges in order to post a new challenge. Just click the ‘challenge’ icon at the top of your feed:

You can then give your challenge a title and description. Like ideas, descriptions are character limited and these are controlled via the main global character limit in settings.

You can also add optional attachments and files to enrich your challenge (maximum file size of 10mb per attachment), in addition to adding categories and tags, just like on ideas. Categories are controlled at a global level in the settings section.