Can you rat someone by a link?

If you mean "rat someone", as in catch them in a act of something undesirable then yes, you can.

An example:

A member of law enforcement finds a black market online. To "rat out" a number of offenders all he has to is post a link to a third party site disguised as a tight deal on firearms or some such item. The site could really be a police web-bot that reads the IP address of all computers that connect and possibly downloading keyloggers to the same computers.

This could count as ratting someone with a link.