Essay Page Type - inserting cotents of a variable

When you create an Essay (Survey) page, it consists of a header textbox and a text entry box (automatically associated with a variable). The text entry box has default instructions in it ("type your text here"). When the user places the cursor in the text entry box the instructions disappear and the text can be entered by the student. But...

I would like to do the following:

(a) Replace the instructions with the content of a variable and

(b) Stop this new text from disappearing when the cursor is placed in the text entry box. In that way students can edit the text in the text entry box and save it in the variable associated with the entry box.

I have tried inserting a variable in the text entry box in the usual way but it does not seem to be interpreted.

Any thoughts?

Thank you

Andrew