At the cost of 90 Uc, you can broadcast a custom message of about 100 characters (3 sentences/lines). The box only allows you to type a sentence then you can't see what you've typed anymore. Note that it won't be submitted if it breaks the ROBLOX content filter and will be cleared.
in order to do this, cross the bridge, then take an immediate left. past the bush there will be a clearing, and sitting on the wall to your left will be a megaphone. click on it to shout