"Tell a Friend" is often used interchangeably with "Forward to a Friend". However, "Forward to a Friend" actually refers to forwarding an email message to a friend, while "Tell a Friend" refers to forwarding a webpage to a friend.

Here's how you would configure the Forward to a Friend link:
<A href='http://www.yoursite.org/site/TellAFriend?msgId=EmailMessageIDHere">Foward to a Friend</A>

The e-mail message ID is visible when creating a new email in the URL.

Here's an example of the URL where the Message ID is 20838.0:
https://www.yoursite.org/admin/CommCenter?ec_id=1141&em_id=20838.0&email=em_edit1

So this is how you'd configure the Forward to a Friend link in that particular example:
<A href='http://www.yoursite.org/site/TellAFriend?msgId=20838.0">Foward to a Friend</A>
Note: Forward to a Friend is only available on regular email messages and not in autoresponders, The campaign must be web viewable in order for the link to work and the link must be placed in the email body rather than the stationary. If placed in the stationary instead of an individual message, the Tell A Friend will not be message specific and it will default to the generic site's Tell a Friend because there will be no message id tied to the link in the stationary. In addition, you cannot specify a Forward to a Friend link for one message inside of a different message. This will result in the link redirecting to the message it was sent from.