<script language="JavaScript">

prompt("What is your name?","Your Name")

</script>

Result: That thing that popped up. Click Here if you missed it.

First you put the prompt command, then a parentheses.  It sounds just like the alert command.  Then comes the quotation marks, the first set is what you want the gray area to say, the second is what you want the text to say.  You always have a comma separate them.  If you wanted the text area to say nothing you would just put nothing in between the quotation marks.