Need a JavaScript script that will help me autofill a form within an iFrame!

I need a script to autofill a form located within an iFrame using JavaScript. The iframe is located on the final page of checkout on Shopify sites, I will give yoj an example. This will be added to a project to help aid in the process of buying scarce goods. My project is being written in Swift, so i need JavaScript to manipulate the webview using the evaluateJavaScript metjod. So the script must autofill the form from the mobile site also.