how to access elements in iframe generated from JS

Question

I have an iframe like this

<iframe
        id="subscribe-form-iframe"
        style="border: none;"
        src=""
></iframe>

I display it on the page only when I need it, using JS:

$('#open_subscribe_form_link').on('click', function (ev) {
        ev.preventDefault();
        var src = 'http://www.myiframesource.com';
        $("#subscribe-form-iframe").attr('src', src);
        $('.subscribe-form-div').slideToggle();
});

this works fine but, how can I access the elements (submit button or change input values) inside the iframe using jquery? I already tried this with no luck.

thanks