JSFiddle - React, Tailwind, and code Playground

HTML

<textarea rows="10" cols="5">10AFFURL</textarea>
    <textarea rows="10" cols="5">20AFFURL</textarea>
        <textarea rows="10" cols="5">30AFFURL</textarea>

JavaScript

$(function(){
    var prodUrl = "test.com";
    $("textarea").each(function(){
        
this.value = this.value.replace("AFFURL",prodUrl );
    });
});