Add message into source file

Hello there!

How can I add a message to WordPress, which is visible only when to see page source? Many big companies have that thing, so when to see site's source, there is link to job offerings page. I tried adding text to index.php page, but it didn't seem to work.

For example, to see what I'm looking for, open up vox.com, view it's source and then you see a big message there.
Regards