Use jQuery to add tabindex to field group attributes

CF-field-group-tabindex.php

//Replace #fld_8768091_1-wrap by the field ID found with developer console.
add_action( 'wp_footer', function(){
	echo "<script>
		jQuery(document).ready(function(){
		    jQuery('#fld_8768091_1-wrap').attr('tabindex','0');
		    jQuery('#fld_9970286_1-wrap').attr('tabindex','1');
		    jQuery('#fld_6009157_1-wrap').attr('tabindex','2');
		});
	  </script>";
}, 10 );