There are a number of ways, one of the simpliest is as follows

The ADSUTIL.VBS file exists in the following directories:

IIS4 = \Winnt\System32\INETSRV\AdminSamples
IIS5 = \Inetpub\AdminScripts

To Set the REALM
D:\Inetpub\AdminScripts>cscript adsutil.vbs SET w3svc/1/realm Chris
Microsoft (R) Windows Script Host Version 5.1 for Windows
Copyright (C) Microsoft Corporation 1996-1999. All rights reserved.

realm                           : (STRING) "Chris"

In the sample above, I am setting the REALM on the default web site. The /1/ in the above command is used to identify the web site to modify.

You can determine any web site # by selecting the web site, and clicking on properties, and on the Web Site tab, click the properties button for logging.

The filename at the bottom of the dialog shows you the # of the web site.

To get the REALM
D:\Inetpub\AdminScripts>cscript adsutil.vbs GET w3svc/1/realm
Microsoft (R) Windows Script Host Version 5.1 for Windows
Copyright (C) Microsoft Corporation 1996-1999. All rights reserved.

realm                           : (STRING) "Chris"