JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" value="1" />
<input type="text" value="2" />
<iframe tabindex="-1" id="iframe1" src=""></iframe>
<iframe tabindex="-1" id="iframe2" src=""></iframe>
<input type="text" value="3" />
<input type="text" value="4" />
<button>Enable iFrame Tabbing</button>

JavaScript

$("button").click(function() {
   document.getElementById("iframe1").tabIndex = 0;
   document.getElementById("iframe2").tabIndex = 0;
});