This is for splunk 7.3.3 enterprise on Win 2012 server.
In order to connect a slave to a license manager, an admin user would normally go to Settings > Licensing and select Change to Slave.
However, my splunk instance has an expired license, and won't allow access to anything from the GUI, giving only options to Add License or Get License. And neither of these options allows the configuration of a slave.
If it was Splunk LIght, I could get a free license, then switch to the license manager. Since it is Enterprise, I do not have that option.