Open IIS Manager on your Web Server and Select the Website for which you want to customize your logging fields. The changes can also be done at the Server level but that depends on the requirement.

Double-click on Logging icon.

Click on Select fields to select or remove any fields that you want in your IIS logs.

To add any custom field, click on the Add Field button as shown below and add the required header. The below example shows how you can get the Client IP information from the X-Forwarded-For Header (XFF) when the Website is hosted on a Server in a Load Balanced environment. The source of this information is in the Request header. The new log file will have an “_x” suffix to it’s name after modification.

The Logs directory shown above is where your Log files are saved. To identify the file name, check the Website ID under Sites on the left pane.
The Log file name format will be “W3SVC<ID>”.

Click on Apply on the Actions Pane on the right to apply the changes.