Bring IPTC Keyword field data into Javascript as a string

I can’t script, however I am guessing that you may need DC:Subject rather than IPTC:Keyword metadata.

This conditional action script does what you require:

Siva's Photoshop Conditional Action: Siva's Photoshop Conditional Action

As does this script:

https://forums.adobe.com/thread/2314586

As an example, if the keyword was “key1” (line 10 below), then run a Photoshop action titled “Molten Lead” in the set “Default Actions” (line 12 below). The action obviously needs to be loaded first, not sure if you can script to ensure that this is the case or not:

#target photoshop;  
if(documents.length){  
if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');  
var xmp = new XMPMeta(activeDocument.xmpMetadata.rawData);   
var shadow = xmp.getProperty(XMPConst.NS_CAMERA_RAW,"ParametricShadowSplit");  
var exposureValue = xmp.getProperty(XMPConst.NS_CAMERA_RAW, "Exposure");   
var keys = getArrayItems(XMPConst.NS_DC,'subject');  
for(var a in keys){  
    //amend the match for your keywod  
    if(keys.toString().match(/key1/)){  
        //things to do if keyword found  
        app.doAction("Molten Lead","Default Actions.atn")  
        break;  
        }    
    }  
};  
function getArrayItems(ns, prop){  
var arrItem=[];  
var items = xmp.countArrayItems(ns, prop);  
            for(var i = 1;i <= items;i++){  
                    arrItem.push(xmp.getArrayItem(ns, prop, i));  
                }  
return arrItem;  
};