What is the parameter for prefixes in Search urls?

[acanuck]

I'm trying to create a custom search through the url.

I've managed to get this to work:
website.com/search.php?query=mykeyword&do=process

but when I try
website.com/search.php?prefixchoice[]=mykeyword&do=process

It does not work in vb 4

Did the parameter for prefix change? and if so what is it now?

Thanks :D