Inno Setup to modify key values in app.config file while running setup.exe - xpath

Question

I need to change the values of some keys in app.config file. I'm planning to use Inno Setup for this task.
I have found excellent tips of using XML parser and xpath selectors in this kind of tasks. However, I don't know what path to use to point to the key & value I want to change.
My app.config looks approximately like this:
<configuration>
<runtime>
</runtime>
<appSettings>
<add key="DefaultUrl" value="http://localhost/Some_application"/>
</appSettings>
</configuration>
So, I want to access the key "DefaultUrl" and change it's "value". How to point there?
$x("//configuration/appSettings[#key='DefaultUrl']")
doesn't seem to be correct since it returns nothing.
EDIT after Mirtheil's comment:
To change the key values I use this procedure:
procedure SaveAttributeValueToXML(const AFileName, APath, AAttribute,
AValue: string);
var
XMLNode: Variant;
XMLDocument: Variant;
begin
XMLDocument := CreateOleObject('Msxml2.DOMDocument.6.0');
try
XMLDocument.async := False;
XMLDocument.load(AFileName);
if (XMLDocument.parseError.errorCode <> 0) then
MsgBox('The XML file could not be parsed. ' +
XMLDocument.parseError.reason, mbError, MB_OK)
else
begin
XMLDocument.setProperty('SelectionLanguage', 'XPath');
XMLNode := XMLDocument.selectSingleNode(APath);
XMLNode.setAttribute(AAttribute, AValue);
XMLDocument.save(AFileName);
end;
except
MsgBox('An error occured!' + #13#10 + GetExceptionMessage,
mbError, MB_OK);
end;
end;
I call it
SaveAttributeValueToXML(Some_Application.exe.config', '//configuration/appSettings[#key=''DefaultUrl'']', 'value', 'https://Server_Name/Other_Application');
This call generates an exception on line
XMLNode.setAttribute(AAttribute, AValue);
Exception message: "Variant is null, cannot invoke"
EDIT 2:
I found the answer. Calling
SaveAttributeValueToXML(Some_Application.exe.config', '//configuration/appSettings/add[#key=''DefaultUrl'']', 'value', 'https://Server_Name/Other_Application');
changes the value of key "DefaultUrl".

Answer 1

I found the answer.
$x("//configuration/appSettings/add[#key='DefaultUrl']")
returns the node I want.