Word doesn't have a placeholder text feature. You could create an AutoText
entry, but it would be a fixed amount of content regardless of text box
size. If you want anything more sophisticated than that you'd need a macro.
if you type =rand() or =rand(2,3) and then hit enter, you get dummy
text (2 paras of 3 sentences each, in the last example). But it will
overspill a text box as it needs to.