I tried this,

I set pResponse As %String for the method output

set pResponse=""
set formatter = ##class(%JSON.Formatter).%New()
set tJSONFormattedString=""
set tSC = formatter.FormatToString(tHttpResponse.Data.Read(),.tJSONFormattedString)
set pResponse = tJSONFormattedString
quit pResponse

It produces this error

ERROR <Ens>ErrGeneral: Quitting with error on Message body 6@AH.AHLIB.Custom.Symedical.Message.PostRelationRequest / 1201174076 because Status 'ERROR #5034: Invalid status code structure ("["_$c(13,10)_" {"_$c(13,10)_" ""RequestedTerm"":{"_$c(13,10)_" ""CatalogIdentifier"":""S_ASC_CARE_BRANCH"","_$c(13,10)_" ""TermSourceCode"":""ASC-BRC-000179"","_$c(13,10)_" ""IncludeDomainCharacteristics"":true"_$c(13,10)_" },"_$c(13,10)_" ""Messages"":["_$c(13,10)_" ],"_$c(13,10)_" ""RelatedItems"":["_$c(13,10)_" {"_$c(13,10)_" ""RelationshipUID"":""1146f07d-9c93-4c4e-82ba-0989fa6a62d9"","_$c(13,10)_" ""RelationshipMnemonic"":""HAS_SITECORE_ID"","_$c(13,10)_" ""CatalogU.....

Response is an array

so my respose class is defined as,

Property Response As list Of AH.AHLIB.Custom.Symedical.Message.PostRelationResponse.response;

class AH.AHLIB.Custom.Symedical.Message.PostRelationResponse.response is defined as,

Parameter %JSONIGNOREINVALIDFIELD = 1;

Property RequestedTerm As AH.AHLIB.Custom.Symedical.Message.PostRelationRequest.termsRequested;

Property Messages As %Library.DynamicArray;

Property RelatedItems As %Library.DynamicArray;

Property SharedRelations As %Library.DynamicArray;
 

Last three properties are arrays, but I don't need their contents. Do I define them right?