Form in popup on mobile

yes – but think of jQuery 3.5.1 : jQuery(window).load(function(){ is deprecated now.
You had to use instead: jQuery(window).on('load', function(){

or here in the code: Link

$(window).on('load', function(){
…

and if it is the syntax is:

(function($){
…
})(jQuery);

you can write every jQuery inside as $: ( f.e. $(‘body’) }

function av_open_inline_popup(){
?>
<script type="text/javascript">
(function($) {
$(window).on('load', function(){
	$('.open-popup-link').addClass('no-scroll');
	$('.open-popup-link').magnificPopup({
		type:'inline',
		midClick: true,
		callbacks: {
			beforeOpen: function () {
				$('body').css("overflow-y", "hidden");
			},
			close: function() {
				$('body').css("overflow-y", "auto");
			},
		},
	});
});
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'av_open_inline_popup');