by ben reynolds.
Following Mary to say that, probably, the professor doesn't want students editing something in a textbox because the prof wants to see how the edited version differs from the unedited. In that case, all you need is a file to download and students then upload the edited version.
But, if the prof is using a textbox, you can simply expand the instructions to say "copy this stuff that is in the assignment description, edit it, and paste it into the textbox."