Hi,
This is not possible from App Inventor or through any other app, as WhatsApp doesn't provide a mechanism to send text to someone without the interaction of the user (to pick a contact to send the message to). If you are interested in sending messages though WhatsApp, have a look at this tutorial here:
If you still prefer a action-less method (that's not at all related to AppInventor), you can use Google Now to send messages using voice. Tap the mic and speak "send a whatsapp message to [my brother] | [any contact] (message).
Eg: Send a whatsapp message to my brother i'll call you in few minutes.
and this will send a message without any further interactions. Other way to implement your evil idea ;) would be to code a custom app that modifies WhatsApp database on a rooted device. Unfortunately, none of the said methods could be accomplished through AppInventor.
Regards.