Property doesn't write when schema isn't present.

app.document.deselectAll();  

  
var count = app.document.visibleThumbnailsLength;  
 app.synchronouseMode=true; 
for ( var i = 0; i < count; i++ ) {  
    if ( app.document.visibleThumbnails[i].name.indexOf("-lr")==-1&&app.document.visibleThumbnails[i].name.split(".")[1]=="pdf") {  
        var selectedFile = app.document.visibleThumbnails[i].spec;  
         app.document.select( app.document.visibleThumbnails[i] );  
         var thumbnailMeta=app.document.visibleThumbnails[i].metadata;
         //var addIPTC_Core=thumbnailMeta.applyMetadataTemplate ("Imaging_Template_1.0","append");
         $.writeln(thumbnailMeta)
        if (ExternalObject.AdobeXMPScript == undefined){
            ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");   
            }
            var myXmpFile = new XMPFile( selectedFile.fsName, XMPConst.UNKNOWN, XMPConst.OPEN_FOR_UPDATE);   
            var myXmp = myXmpFile.getXMP();   

So if I use this library I am only able to write to this file if that schema exists. Right now I have a PDF that just has some custom metatdata and I want to add some ITPC_Core metat data to it but That schema doesn't exist. So I either need to figure out how to write to the custom schema or create a new ITPC_Core schema and then edit that. Any help is appreciated.

function setXMPValue (xmpType,propertyValid,propertyToSet,propertyValue,propertyFlag){
    
         // var Email = myXmp.getStructField( XMPConst.NS_IPTC_CORE, "CreatorContactInfo",XMPConst.NS_IPTC_CORE, "CiEmailWork");///gets the value of the structure
         try{
            if(propertyValid){
                    myXmp.setArrayItem(xmpType, propertyToSet,1,propertyValue); 
                    }
               else{
                    if(propertyFlag=="field"){
                        //if the property is nested and does not exists already we have to setStructField which requires a different constructor
                        var setAField= myXmp.setStructField( XMPConst.NS_IPTC_CORE, "CreatorContactInfo",XMPConst.NS_IPTC_CORE, "CiEmailWork",propertyValue);///sets the value of the structure for field
                        }
                    else{
                        myXmp.appendArrayItem(xmpType, propertyToSet, propertyValue, 0, XMPConst.PROP_IS_ARRAY); 
                        }
                    
                    
                    }
                }
            catch(err){
                $.writeln(err)
                }
    
    }