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This method copies across everything in an existing broadcast, including the message content and the audience query. This can therefore be a quick way to create something similar again. To copy an existing broadcast, click the copy icon 
next to the item in the list.
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This will create a new broadcast in the list. From here you can edit the broadcast by clicking the 
edit icon. From here, check the various stages in the editor to edit your broadcast (Setup, Audience, Message, Testing, Sending, Reporting). For more information on these, click on the links.
Create a new broadcast from scratch
Or, to To create a new broadcast from scratch, click the orange Create Broadcast button.
You will be taken to the Setup screen where you can decide on the name of the broadcast and type of message.
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Read more about the Setup screen here.