I have a button like this
try this
https://jsfiddle.net/y64qyjzo/
Update: added a timer to allow the user to notice the modal:
$(document).ready(function() {
$(".render").click(function(evt) {
evt.preventDefault();
$('#render_page').modal('show');
setTimeout(function() {
$('#testForm').submit();
}, 1000)
});
});
(add the id 'testForm' to the form)
https://jsfiddle.net/y64qyjzo/3