insert javascript

keep in mind that the

jQuery(window).load(function(){

is deprecated, you now have to use:

jQuery(window).on('load', function(){

hm ?

some thoughts on custom scripts via functions.php

you can do that:

function custom_script(){
?>
<script type="text/javascript">
(function($) {
  $(window).on('load', function(){
    $('.open-popup-link').trigger('click');
  });
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'custom_script');

Maybe – i think it will be enough to just wait til DOM is loaded and not the whole page
so this could therefore be sufficient

function custom_script(){
?>
<script type="text/javascript">
window.addEventListener("DOMContentLoaded", function () { 
	(function($) {
	    $('.open-popup-link').trigger('click');
	})(jQuery);
});
</script>
<?php
}
add_action('wp_footer', 'custom_script');

But
did you really got that class on the anchor?
Lets say you got a button as trigger for the inline popup loading. And you gave a custom class to that button element – the class will be at the button wrapper – not the anchor!

so the selctor must be different:

function custom_script(){
?>
<script type="text/javascript">
window.addEventListener("DOMContentLoaded", function () { 
	(function($) {
	    $('.open-popup-link a').trigger('click');
	})(jQuery);
});
</script>
<?php
}
add_action('wp_footer', 'custom_script');