How to make spotlight index specific files immediately?
Something is messed up. Other than when your system has no index, spotlight indexes as the file is written - literally its part of the process writing the data. 

The only exception I’m aware of is you can purge spotlight or have a new install and the system queues an index of all files. My recollection is that index only lags for files that were on the drive and new files are indexed immediately. 

I don’t thing there’s a setting to change this so instead of wondering how to speed it up, run mddiagnose and work with support to see why your system isn’t caught up. (Or dig in to the diagnostics to see if you can spot the issue(s))