How do you credit copied code?

Are comments giving the source good enough?
like this:
//I coped the following function's code from Stack Overflow. Here is the original source:
//[original url goes here]
Or if you copy something that's on github, do you have to fork it, or can you just copy it and also do the comment thing? Can you just message someone and ask, or can you do it without asking?

Sasha went to job interview
Examiner asks: show me git hub!
But Sasha can not
Sasha killed hamster and is banned from Git Hub !!
Sasha is fired and dies poor
hahahahahaha