very simple!!!!
surprised it's not already in use on all themes!
so, you need to know what class you have named your button. for instance; with a theme using bootstrap, it's called ".btn"
in bender it's called ".ui-button" in modern it's called nothing, so you will need to class it.
if you do not know what I'm referring to it is this:
<button class="****" type="submit"><?php _e('Publish', 'modern'); ?></button>
now open the item-post.php and paste the following just before your body closing tag </body>
<script> $(document).ready(function() {
$(".btn").click(function() {
$("*").css("cursor", "wait");
});
});</script>
finally replace the .btn with the proper button class as explained above, keeping the quotes.