You would have to pass the value inside the embedding code itself. This is a sample embed code of a form:
<iframe id="miniExtIframe-jV8YFoL846PqKq48Uxxl" height="800" src="https://web.miniextensions.com/jV8YFoL846PqKq48Uxxl"></iframe><script id="embed-script-id" type="text/javascript" src="https://web.miniextensions.com/statics/embed.js?miniExtIframeId=miniExtIframe-jV8YFoL846PqKq48Uxxl"></script>
You will see the URL of the form in the first src parameter in the embed code. You have to append the prefill in this parameter.