Step 1: In the channel, select Format Expand button under the box where you type your message.
Step 2: At the top of box that appears, click the arrow next to New conversation > Announcement.
Step 3: Type a headline, change its background color, or add a background image for the headline area—lots of options!
Step 4: Type the topic of your announcement in the Add a subhead area, then the body of your message, and select Send Send button.