How can i add backlinks in a website source code?

I've scanned with a software some websites who appears 1st in google , and they have backlinks on other sites source code , and the web-administrators don't know about the source code link. They are like this: <div style="display: none;">code link here</div> and they are visible only in the source code.

If Google can't see it on the webpage then it does nothing.

Yeah google needs to see it

Trust me , it works ! Google does not have to see the link
open this site discomobile . net, and in source you can see: < div style="display: none;">the code link with title jocuri online</div> and etc

i don't understant how hacked this site and put her backlink. i want hacked a website too....