Is it possible to use a portfolio layout but change the links from Projects to external URLs?

I am very new to this, so sorry if I'm not very clear about my problem.

So. I made a gallery page (a portfolio) to showcase projects:

and inside, there are what's called 'Projects':

these projects are individual pages.

When I'm browsing the gallery and pressing any of these links, it is taking me to the project page, but instead, I wish to relink them to an external URL site. but it's not possible. It is only letting me relink to internal sites via slug. I tried doing URL Mappings (I made sure I wrote it right):

but it doesn't work.
I read that it might work if I could disable these pages, but there isn't any option to.

TL;DR:
I'm looking for a way to relink project pages to external sites (like youtube). how do?

THX alot!

URL Mapping for 404 pages only

You mean

When users on Portfolio LIST page >> Click on each sub page >> It will redirect to external site?

Hi tuan!
Yes, that's exactly what I mean. My programmer friend managed to help me out with an injected java script to actually redirect the links to external ones! So I'm good. I'd share his code but I think I need his permission.

Also, wanted to say thanks alot tuan, I've been using your solutions for however long i'm making my website.

I also use JavaScript to achieve this.

Or recreate portfolio with Blog Page then use Summary BLock to pull out posts & use Source URL to redirect to external link.