Get Custom Field value from Post ID

Hello,

I’m trying to grab the value from a custom field from a a specific post id.

Basically, when someone visits:
https://www.mydomain.com/pages/code/source/index.php?id=(thepostid)

The value of the custom field named “source_code” from the post id that’s the parameter and display it on screen.

I’ve tried to do this myself, but run into trouble.

Any ideas how I could do this?

Thanks in advance.