JSFiddle - React, Tailwind, and code Playground

HTML

<a href="http://gog.com/en/gamecard/baldurs_gate_the_original_saga/">myWikiLink</a>

<a href="http://www.google.com">nonGogLink</a>
<a href="http://www.amazon.com">amazon</a>

JavaScript

$("a").click(function() {
    addAffiliate(this);
});

myCode = "?pp=708a77db476d737e54b8bf4663fc79b346d696d2";
myAmazonCode = "?tag=shihac-20"
    function addAffiliate(link) {
        alert("enterting script: " + link.href);
        if ((link.href).indexOf("gog.com") > -1 && (link.href).indexOf(myCode) < 0) {
                link.href = link.href + myCode;
        }else if((link.href).indexOf("amazon.com") > -1 && (link.href).indexOf(myAmazonCode) < 0){
                link.href = link.href + myAmazonCode;   
        }
            alert(link.href);
        return true;
    }