To process a string in the response message, cast the Entity field in the response object to an OpenEdge.Core.String object. For example:

USING OpenEdge.Net.HTTP.*.
USING OpenEdge.Net.URI.
USING Progress.Json.ObjectModel.*.


DEFINE VARIABLE oClient AS IHttpClient NO-UNDO.
DEFINE VARIABLE oURI AS URI NO-UNDO.
DEFINE VARIABLE oRequest AS IHttpRequest NO-UNDO. 
DEFINE VARIABLE OResponse AS IHttpResponse NO-UNDO.
DEFINE VARIABLE ResponseString AS OpenEdge.Core.String NO-UNDO.

//Build a request
oURI = URI:Parse("http ://httpbin.org/robots.txt").
oRequest = RequestBuilder:GET(oURI):Request.

//Execute a request
oClient = ClientBuilder:Build():Client.
oResponse = oClient:Execute(oRequest).

//Process the response
IF oResponse:StatusCode <> 200 THEN
    RETURN ERROR "Request Error: " + STRING(oResponse:StatusCode).
ELSE
    ResponseString = CAST(oResponse:Entity, OpenEdge.Core.String).
    MESSAGE ResponseString.