Hello Chohan,
unfortunately this is not a standard function.
The solution tell a friend, which Andy suggested, allows the candidate to send the posting to someone else so he can apply to it. But this is only available for external candidates. In the job searches for internal candidates (=employees) the function is not available. Also an employee sees internal jobs which may be not available / published to external candidates which would make it impossible for the external person to apply. When using tell a friend the information who refered the job to the one who is applying is not stored and would be not available for referral programs.
The only standard option I am currently aware of is the application source. The source of application has a fixed defined type for employee referral. If an external candidate applies he can select this entry and the defined key has a trigger to display an input field where he can enter the employee. The field is a free text field so technical processing of its content might be tricky but at least the recruiter can get the information.
Kind Regards
Roman