Use the ReplyRecipients Collection of the mail item object.

Here is a sample:

#Define olMailItem	0
oOutlook = createobject("outlook.application")
oNameSpace = oOutlook.GetNameSpace("MAPI")
oMailItem = oOutLook.CreateItem(olMailItem)
With oMailItem
   .Subject = "My Message"
   .To = "xyz@foo.com"
   .ReplyRecipients.add('foo@abc.com') && specify the reply address here
   .Send
EndWith