OnBeforeBind is a TNotifyEvent property that represents the event handler signalled prior to binding the socket handle using the Host and Port for the connection. 

Applications must assign a procedure to the event handler to allow responding to the event notification. 

OnBeforeBind is assigned to the event handler of the same name found in the Socket property when an IOHandler is assigned for the client connection. Assigning a new procedure to OnBeforeBind will cause the same procedure to be assigned to the event handler in an existing IOHandler. 

Use OnAfterBind to respond to the event notification that occurs after binding the socket handle for the connection.

TIdTCPClientCustom.OnAfterBind 

TIdTCPConnection.Socket 

TIdTCPConnection.IOHandler TIdIOHanderSocket.OnBeforeBind