I am using BAInsight Custom connectors to crawl/index SQL Databases in my SharePoint On-Prem environment. We also have SharePoint Online tenant and I have enabled hybrid federated search results in SharePoint Server. I now want to also display the results from custom connectors to be displayed in SharePoint Online. I tried setting up a new result source but am unable to get all managed properties that are there in SharePoint Server. Any pointers / help.
Have you considered using Hybrid Search rather than query federation? You can crawl all of your on-prem content and build one index in SPO. Then you manage the result sources and Managed Properties in SPO.
If you stay with query federation, you can use the on-prem managed properties in your queries of on-prem content, but you will have to write them by hand since they are not "known" to SPO.