How to Set Survey Values

I'm embedding a simple chat button into a site where users are already logged on, therefore I already know their name and email.

In the Pre-Chat survey there are questions for Name and E-Mail.

I want to use Javascript to set these questions to the known values but cannot find the documentation for it.  Can someone point me to the correct place?

My button is just defined like this:

<script type="text/javascript" charset="UTF-8" src="https://server.iad.liveperson.net/hc/#######/?cmd=mTagRepstate&site=7357772&buttonID=1&divID=lpButDivID-1386011190858&bt=1&c=1"></script>