Here it is: How to find where to edit this and that
Look up for 'textarea' in miniBB folder, most likely you will find the only one template holding message area, it is main_post_form.html.
Providing placeholder would be truly nice idea for your approach. There is a way to put straight "physical" text in the message area, but it should be handled on the coding level, because this template is also used in the other places - private messaging, editing message. Having some pre-defined text in it is impossible for these places.