Web Api with POST operation isn't working and throws 404 error

Trying to create a Web Api with POST operation with List of strings {source: ["","",""] }as input in body. (But the appian documentation states, body should have something of form "id=7&name=Acme+Corporation&firstPurchaseDate=2016-03-12", this is like list of query params built as string to attach to the api endpoint.) Testing in Postman app. 

It returns 404 not found error. But other endpoints of GET type are working fine. Could it be the problem of passing input List of strings in body instead of in form-data like mentioned in below URL {id:"7", name:"Acme Corporation", firstPurchaseDate:"2016-03-12"}

https://docs.appian.com/suite/help/18.1/Designing_Web_APIs.html 

This link also clearly mentions " If the same request did not contain a content type header or if it had a value other than application/x-www-form-urlencoded, the formData field of http!request would have been null. "

But, need to pass a json list of arrays as mentioned above in POST request to retrieve a bunch of information based on these list of source items. don't want to pass anything in URL or as URL encoded type.

Please advise how to get rid of this error.

Also please explain what is a "PATH SEGMENT"? It looks like taking single values like query params to form an endpoint. In what scenarios we need to map this PATH segment.