JSFiddle - React, Tailwind, and code Playground

HTML

<iframe id="photopea" src="https://www.photopea.com?p=%7B%22files%22:%5B%22https://www.photopea.com/api/img/pug.png%22%5D,%22environment%22:%7B%7D%7D" width="630" height="500" />

JavaScript

var cnt = 0, wnd = document.getElementById("photopea").contentWindow;
window.addEventListener("message", onMessage);

function onMessage(e) {   
	cnt++;
	if(cnt==1) {  /* Photopea loaded! */  }
	if(cnt==2) {  /* Image loaded!  Run some script! */
		var x = "app.activeDocument.resizeCanvas(200,200,AnchorPosition.MIDDLECENTER);";
		wnd.postMessage(x, "*");
	}
}