I haven't tested the code below, but in your subclass you can override MakeRequest(), call the original superclass version of MakeRequest() and then you should be able to set the header before returning from your custom method.

Something like this:

Method MakeRequest(pRequestMethod As %String, pPayload, pRequestPath As %String, pQueryString As %String, ByRef pHeaders) As %RegisteredObject
{
    set ..%HttpRequest = ##super(pRequestMethod, pPayload, pRequestPath, pQueryString, .pHeaders)
    do ..%HttpRequest.SetHeader("Some Header Name", "Some Header Value")
    return ..%HttpRequest
}

Is the problematic class particularly large?

I agree with Vic that this is probably not worth the complexity. The rule definitions are stored as XML -- to change them you'll need to fetch the XML, modify it, then save the changed XML and re-compile the class. It would look something like this:

    set tMyRuleClassName="My.RoutingRuleClass"
    
    &SQL(select %id into :tXDataID from %Dictionary.XDataDefinition where parent = :tMyRuleClassName and Name='RuleDefinition')
    
    if (SQLCODE > 0) {
        // Query returned no results. Do some error handling
        quit
    }

    set tRuleDefinition=##class(%Dictionary.XDataDefinition).%OpenId(tXDataID)

    /* This stream contains the raw XML of the RuleDefinition. It looks like this:
    <ruleDefinition alias="" context="EnsLib.HL7.MsgRouter.RoutingEngine" production="TESTINGPKG.FoundationProduction">
        <ruleSet name="" effectiveBegin="" effectiveEnd="">
            <rule name="">
                <when condition="1">
                    <send transform="" target="HL7.File.Out"/>
                    <return/>
                </when>
            </rule>
        </ruleSet>
    </ruleDefinition>
    */
    set tRuleDefinitionXMLStream=##class(%Stream.GlobalCharacter).%New()
    do tRuleDefinitionXMLStream.CopyFrom(tRuleDefinition.Data)

    // Use XML classes to manipulate the stream
    // (docs about XML classes: https://docs.intersystems.com/irislatest/csp/docbook/DocBook.UI.Page.cls?KEY=GXML_intro)
    // ...
    // ...
    // ...
    // Ok, done making changes to the stream

    // Copy the changed XML back into the RuleDefinition
    do tRuleDefinition.Data.CopyFrom(tRuleDefinitionXMLStream)

    // Save the changees to the rule definition XData
    set tSC=tRuleDefinition.%Save()
    // Check if tSC indicates an error and do something about it
    if $$$ISERR(tSC) {
        // Do some error handling
    }

    // Recompile the rule class
    set tSC=$System.OBJ.Compile(tMyRuleClassName,"cuk",.errorlog,1)

    // Check if tSC indicates an error and do something about it
    if $$$ISERR(tSC) {
        // Do some error handling
    }

To get the gradient, you could assign a background image to a <container>.

You'll probably want to check with the vendor to confirm where they expect to see it. One possibility is to pass it to GetAccessTokenClient as a property.

https://docs.intersystems.com/irisforhealthlatest/csp/docbook/Doc.View.c...

Also, this sample assumes that they're using the client credentials grant type. This is the most likely option, but it would be worth confirming.

I like to use local multidimensional arrays as Dmitriy suggested.

Also, array objects are key/value based -- iterating through the keys always occurs in sorted order.

Here's a cheat sheet I put together on creating an OAuth client definition:

Go to System Administration >> Security >> OAuth 2.0 >> Client

Choose “Create Server Description”

Enter the endpoint URL provided for the OAuth server and select the TLS config you created for the OAuth server. Click “Discover and Save”. It should populate details about the OAuth server.

After saving, a new server entry will appear. Click “Client Configurations” and then “Create Client Configuration”

Enter the name details, select the TLS config, and choose Client Credentials as the grant type. The redirect URL won’t actually be used, but you’ll need to enter something for hostname anyway.

Keep track of what you enter for “Application Name”. You’ll need to put this in your custom code.

Switch to the “Client Credentials” tab and enter the Client ID and Client Secret you were given.

I think for Issuer endpoint you need to include the protocol:

https://api.veritystream.cloud/services/oauth

This snippet uses the built-in OAuth client to request a token and then add it to the HTTP header.

You'll need to configure an entry for the OAuth server with a sub-entry for this specific client:
https://docs.intersystems.com/healthconnectlatest/csp/docbook/Doc.View.c...

In the case of the sample code below, the client I configured is named TestClientA.

    set isAuth=##class(%SYS.OAuth2.AccessToken).IsAuthorized("TestClientA",,"*",.accessToken,.idtoken,.responseProperties,.error)

    $$$TRACE("isAuth:"_isAuth)

    if 'isAuth {
        set tSC=##class(%SYS.OAuth2.Authorization).GetAccessTokenClient("TestClientA", "*",, .error)        
        set isAuth=##class(%SYS.OAuth2.AccessToken).IsAuthorized("TestClientA",,"*",.accessToken,.idtoken,.responseProperties,.error)

        $$$TRACE("isAuth2:"_isAuth)
        
        if 'isAuth {
            quit $$$ERROR(5001,"Problem authenticating")
        }
    }

    $$$TRACE("access_token: "_accessToken)
    $$$TRACE("expires_in: "_responseProperties("expires_in"))
    $$$TRACE("scope: "_responseProperties("scope"))
    $$$TRACE("token_type: "_responseProperties("token_type"))
    
    s tSC=pRequest.HTTPHeaders.SetAt("Bearer "_accessToken,"Authorization")

In your REST operation you can just do this. In this case the header name is "Authorization" but you can change it to whatever is required.

s tSC=pRequest.HTTPHeaders.SetAt(tAuthInfo,"Authorization")

Please post the source code for ITB.HL7.BS.XMLService so we can take a look.

One thing to check: is GetAck declared as a "Method" or as a "ClassMethod"? If it is declared as a "Method" then that is the problem. The syntax you're using to call it is class method syntax, not instance method syntax.