2013 – How to connect ContentSource to ResultSource

I have a local, on-premise dev installation of SharePoint 2013 standard edition.

I have created a new ContentSource to crawl a third party API. I can see it crawled APIs successfully. I can even search it with – /_api/search/query?querytext='some-text'

But I’m not able to get any result in Site search. What I know is that to get that Site search to work, we need to have result sources configured.

So my question is how to connect ContentSource with ResultSource? or I’m missing something here?