@abhinavg first I would recommend reading DreamFactory guide to HTTP services.
The base URL for your service really depends on the service itself, typically I use the minimal URL required to use API. As an example lets take a look at NASA API.
The base URL is: api.nasa.gov/planetary/apod
And the example query shown: api.nasa.gov/planetary/apod?api_key=DEMO_KEY
The '?' shown starts the query as shown the parameter here is 'api_key'.
The 'api_key' parameter is configurable inside DreamFactory as described in the DreamFactory docs.
For creating your Swagger I've found the easiest process to be testing the API using a REST client like Postman then saving my successful REST API calls to Postman collections then having this handy tool transform the collection in to Swagger for me. It may require some minimal tweaking to work inside DreamFactory but is much simpler then building from scratch. Hope this helps!