Creates a new NaturalMotion message.
startImmediately: If set to true, the character will perform the message the moment it receives it by GIVE_PED_NM_MESSAGE. If false, the Ped will get the message but won't perform it yet. While it's a boolean value, if negative, the message will not be initialized.
messageId: The ID of the NaturalMotion message.
If a message already exists, this function does nothing. A message exists until the point it has been successfully dispatched by GIVE_PED_NM_MESSAGE.