As per documentation http://www.splunk.com/base/Documentation/latest/Developer/CustomizeLogin :
Add the following key to the [settings] stanza in web.conf in $SPLUNK_HOME/etc/system/local/:
login_content = This is a <b>production server</b>. If you are running some expensive searches, try one of the following servers:
<a href="http://server1:8000">server1</a> or <a href="http://server2:8080">server2</a>
For larger amounts of HTML I found it useful to format multi-line with \, e.g:
login_content = This is a <b>production server</b>. \
If you are running some expensive searches, try one of the following servers:\
<a href="http://server1:8000">server1</a> or <a href="http://server2:8080">server2</a>