value=<%=Replace(Request.QueryString("parm")," ","%20")%>

Вроде так.

Да, JS... Я на VBS привел. Вот про JS:

Microsoft® JScript®
replace Method Language Reference
Version 3


See Also Applies To


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Description
Returns a copy of a string with text replaced using a regular expression.
Syntax
stringObj.replace(rgExp, replaceText)

The replace method syntax has these parts:

Part Description
stringObj Required. The String object or literal on which to perform the replace. This object is not modified by the replace method.
rgExp Required. A Regular Expression object describing what to search for.
replaceText Required. A String object or literal containing the text to replace for every successful match of rgExp in stringObj.


Remarks
The result of the replace method is a copy of stringObj after all replacements have been made.
The method updates the contents of the RegExp object.

The following example illustrates the use of the replace method:


function ReplaceDemo()
{
var r, re;
var s = "The quick brown fox jumped over the lazy yellow dog.";
re = /fox/i;
r = s.replace(re, "pig");
return(r);
}
In addition, the replace method can also replace subexpressions in the pattern. The following example swaps each pair of words in the string:
function ReplaceDemo()
{
var r, re;
var s = "The quick brown fox jumped over the lazy yellow dog.";
re = /(\S+)(\s+)(\S+)/g;
r = s.replace(re, "$3$2$1"); // Swap each pair of words.
return(r);
}

Раз метод формы - post, тогда не request.querystring, а request.form