You can only have one auto message per channel, and the interval must be at least 10 minutes.

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Creating an auto message

You can create an auto message with the timer add command.
The interval parameter must use the proper format, you can learn more here.
The message parameter can be raw text or an embed with dynamic variables.
,timer add (channel) (interval) (message)

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Removing an auto message

You can remove an auto message with the timer remove command. 
,timer remove (channel)