NOTE: .contents() does not work if iframe’s src points to cross domain page. More: Cross domain iframe issue and Get DOM content of cross-domain iframe
//execute code after DOM is ready
$(function() {
//find iframe
let iframe = $('#main_iframe');
//find button inside iframe
let button = iframe.contents().find('#main_button');
//trigger button click
button.trigger("click");
});
<!--Add jQuery library-->
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
If you need to manipulate another page inside iframe, search for official API or GET parameters.
Example: Youtube Embed Video, Google Maps Embed Api