Interesting Mini Project - Need piece of code to prefill into a field and click the submit button

I have a security project to submit and I have the idea but don't know how to put it into code, so I need your help. Scenario : I have an iframe that contains an input field and a submit button. My task is to demonstrate that I can load the iframe with opacity=0 and "PreFill" that field and click the "Submit" button without showing the actual iframe to the visitor of the website. My idea is that we show user something else , like a fake captcha box and when they enter captcha and press submit button, The task actually gets done on the hidden iframe instead, without the user knowing. The description is a bit long but I am sure that once you understand it, it will be very easy to code. I have had a very similar project in the past where I was doing the same but without an input field, and once the guy understood it, it took him no time to make the perfect code. Discuss this in PM with me.