Create an article in the "Alerts" folder. The title/subject will not show but the content of the article will. Keep in mind it is a banner across the top of the front page of the helpdesk. Don't put a full article, a sentence maybe two, and link to relevant info.

To make it show and choose the color add a "Tag" on the right: 

Choose one of these tags. Info is a blue banner, warning is a yellow banner, danger is a red banner, and emergency alerts is a white banner.

Publish the article and it will show up. I recommend putting a review date that is relevant to when you want the banner to go away. Unpublish or delete to remove the banner.