vb.org Archive - View Single Post - Warning when leaving to external site

How can I make it like League of Legends' has their external warning set up?
Here is what I mean;
http://na.leagueoflegends.com/board/....php?t=2910177
if you go to that URL, you will see (3rd post) a link. click it and you will see the warning.
You post your link that is NOT on your board.
You click the link
You are leaving blahblah.com!
Press yes to continue, press no to go back
if they press yes, it brings them to the external url. if they press no, it brings them back to our forums (or the page before, whichever)

what is the simplest way to do this? Is there a mod for this?

X vBulletin 3.8.12 by vBS Debug Information
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X vBulletin 3.8.12 by vBS Debug Information

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X vBulletin 3.8.12 by vBS Debug Information
Page Generation 0.01066 seconds Memory Usage 1,758KB Queries Executed 11 (?)
More Information
Template Usage: (1)SHOWTHREAD_SHOWPOST (1)ad_footer_end (1)ad_footer_start (1)ad_header_end (1)ad_header_logo (1)ad_navbar_below (1)footer (1)gobutton (1)header (1)headinclude (6)option (1)post_thanks_box (1)post_thanks_button (1)post_thanks_javascript (1)post_thanks_navbar_search (1)post_thanks_postbit_info (1)postbit (1)postbit_onlinestatus (1)postbit_wrapper (1)spacer_close (1)spacer_open Phrase Groups Available: global postbit reputationlevel showthread	Included Files: ./showpost.php ./global.php ./includes/init.php ./includes/class_core.php ./includes/config.php ./includes/functions.php ./includes/class_hook.php ./includes/modsystem_functions.php ./includes/functions_bigthree.php ./includes/class_postbit.php ./includes/class_bbcode.php ./includes/functions_reputation.php ./includes/functions_post_thanks.php Hooks Called: init_startup init_startup_session_setup_start init_startup_session_setup_complete cache_permissions fetch_postinfo_query fetch_postinfo fetch_threadinfo_query fetch_threadinfo fetch_foruminfo style_fetch cache_templates global_start parse_templates global_setup_complete showpost_start bbcode_fetch_tags bbcode_create postbit_factory showpost_post postbit_display_start post_thanks_function_post_thanks_off_start post_thanks_function_post_thanks_off_end post_thanks_function_fetch_thanks_start post_thanks_function_fetch_thanks_end post_thanks_function_thanked_already_start post_thanks_function_thanked_already_end fetch_musername postbit_imicons bbcode_parse_start bbcode_parse_complete_precache bbcode_parse_complete postbit_display_complete post_thanks_function_can_thank_this_post_start showpost_complete
Messages: