ReOpen popup 123 when form is submitted. This is needed when the form doesn't support ajax submission. The $_POST key & value will vary based on your form and will need to be replaced.

function.php

<?php
add_action( 'wp_footer', 'pum14_popup_reg_form_check', 1000 );
function pum14_popup_reg_form_check() {
	if ( isset( $_POST['form_id'] ) && $_POST['form_id'] == 'my_form' ) {
		?>
		<script type="text/javascript">
			PUM.open(123);
		</script>
		<?php
	}
}