Thanks Yuri, this is really helpful. I was doing something similar but you have been faster :) 

Hello, it would be interesting to try the beta test of the exam. Is there any specific documentation on the topics listed or material to study to prepare for the exam? 

Hi, it's been quite a while since this question but yesterday I had the same problem and I'd like to share how I managed to solve it.

First of all, I have created a simple unauthenticated Web Application where the dispatch class it the EnsLib.REST.GenericService in order to user the simpler and cleaner architecture.

Then, after processing the HTTP request, I generate the HTTP response through a Business Process where I have declared the following parameters:

Parameter CONTENTTYPEJSON As STRING = "application/json";
Parameter HTTP200OK As STRING = "HTTP/1.1 200 OK";
Parameter HTTP202ACCEPTED As STRING = "HTTP/1.1 202 Accepted";
Parameter HTTP400BADREQUEST As STRING = "HTTP/1.1 400 Bad Request";
Parameter HTTP500INTERNALSERVERERROR As STRING = "HTTP/1.1 500 Internal Server Error";

It is important that the parameters related to the status line starts with "HTTP/1.1", since it is this statement that enable the return of the correct HTTP status.

Finally, I generate the response as shown:

Set ContentType = ..#CONTENTTYPEJSON
Set StatusLine = ..#HTTP500INTERNALSERVERERROR
Set HTTPStatus = 500

; Format the json with the correct indentation
Set jsonFormatter = ##class(%JSON.Formatter).%New()
Set pStream = ##class(%Stream.GlobalCharacter).%New()
Set sc = jsonFormatter.FormatToStream(json, .pStream)

; Generate an HTTP response message
Set pResponse = ##class(EnsLib.HTTP.GenericMessage).%New(pStream)

; Add headers to the HTTP response
Set sc = pResponse.HTTPHeaders.SetAt(StatusLine,"StatusLine")
Set sc = pResponse.HTTPHeaders.SetAt(HTTPStatus,"StatusCode")
Set sc = pResponse.HTTPHeaders.SetAt(ContentType, "Content-Type")

; I tested it with the following HTTP codes either and it works: 
;   202 / ..#HTTP202ACCEPTED
;   200 / ..#HTTP200OK
;   etc...

I tested with HTTP statuses 200, 202 and 500, but I'm pretty sure it will work with other statuses as well.