Hi Ann,
I believe you are going to have webserver(eg: Apache) to front end the IDM servers where you will be creating Virtual hosts. Within this you can have a proxy forward rule to take care of this. something like,
RewriteCond %{REQUEST_URI} ^/$
RewriteRule ^/(.*) https://idm.abc.com/iam/im/identityEnv [R,NC,L]
This will forward your friendly LB url -
https://idm.abc.com
to
https://idm.abc.com/iam/im/identityEnv
Hope this helps !!!.
Regards
Ashok