How to write xmp data

I'm not sure what exactly you want to write, but here's an example of writing some data to a custom (for more complexity) namespace:

main();

function main() {
    var prop = "MyProperty";
    var txt = "Blah-blah-blah";
    addMetaData(prop, txt);
}

function addMetaData(prop, txt) {
    if (!ExternalObject.AdobeXMPScript) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");  
    var xmp = new XMPMeta(app.activeDocument.xmpMetadata.rawData);
    var theNamespace = "http://ns.xinet.com/ns/xinetschema#";
    var thePrefix = "xwnv:";
    XMPMeta.registerNamespace(theNamespace, thePrefix);
    xmp.setProperty(theNamespace, prop, txt);
    app.activeDocument.xmpMetadata.rawData = xmp.serialize();
}

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