Read part of XMP metadata other than info

I've added a extra node to the XMP of my PDF see screenshot from within InDesign …

And now I want to read this out from a javascript.
So I tried to read the metadata with the code

var meta = this.metadata;
var myXMPData = new XML(meta);
myrdf = new Namespace("http://www.w3.org/1999/02/22-rdf-syntax-ns#");
p = myXMPData.myrdf::RDF.myrdf::Description;

But then I'm stuck to get the specific pdfx:PX part.
I can see it in the variable p but have no idea how to get to this pdfx:PX content in a variable to further process it's content.

Can someone help me out?

Any help will be appreciated with a big Thank You!