You need to give an id to the iFrame and try something like this:
runtime.globalVars.New=Myiframe.contentWindow.document.getElementByClassName(...
.
If this doesn't work, you can create a JS function inside the iframe, which will retrieve the value from your HTLM element and return it. And then call that function from Construct:
runtime.globalVars.New=Myiframe.contentWindow.functionName()