Go to your opensolr index control panel.
You should find a list with all your opensolr indexes here
Click on the index you want to restrict
Go to the SECURITY tab
Find out which IP address is your 3-rd party going to connect from
Remove any all is reaching / or all is reaching /SOMETHING from the IP access list
Add the 3-rd party’s IP address to /select -> since that’s the main solr endpoint for reading.
Add any other IPs for /update or simply to / which is all endpoints
That will limit the 3-rd pary to only access /select, while other ips you’ll setup for / or /update or whatever, will have access to all those others.