To make setting this action up a bit easier, would it be possible to add a check box labeled something like "Return Stores Input", or "Disable Carriage Return" to handle this automatically? There would then also be the usual field in the action to select what event to send when return is hit.
I ask only because most people by default expect to be able to just hit the enter key when they are done typing, regardless of whether or not there is an additionional "done" button. Also, this would be the default behaviour most of the time, with the additional lines usually only being used for things like word processing, not something commonly found in games. Even in chat, text should just wrap around on long messages with the Return key only being used to send the message.
Just a thought