Suppose you have your JSON in a stream StreamFHIR, then:

Set BundleObject=##class(HS.FHIR.DTL.vR4.Model.Resource.Bundle).FromJSON(StreamFHIR,"vR4")

If you want ONLY the first entry, then:

Set FirstEntry=BundleObject.entry.GetAt(1)

First I'd suggest not to use a persistent class "linked" to a message (Ens.Response in this case), Supplier  is linked by your message class in this case.
If you do it, you will definitely create "orphaned" persistent objects when you purge, unless you add some logic (like triggers and callbacks) to delete the "linked" persistent objects when a message is purged.

To avoid this (when possible) a serial class is preferred.

So, the Supplier class would be:

Class Community.App.Msg.Supplier Extends (%SerialObject, %XML.Adaptor)
{
Property row As%Integer;
}

As for the message class:

Class Community.App.Msg.Suppliers Extends Ens.Response
{

Property Supplier As list Of Community.App.Msg.Supplier(XMLPROJECTION = "ELEMENT");ClassMethod test() As%Status
{
    set XmlString="<Suppliers><Supplier><row>1</row></Supplier><Supplier><row>2</row></Supplier></Suppliers>"set reader = ##class(%XML.Reader).%New()
    $$$ThrowOnError(reader.OpenString(XmlString))
    do reader.CorrelateRoot("Community.App.Msg.Suppliers")
    do reader.Next(.Suppliers, .tSC)
    do Suppliers.XMLExport(,",indent")
    quit tSC
}
}

For simplicity I modified the sample to be a classmethod.
When the test() method is run the output is:

EPTEST>d ##class(Community.App.Msg.Suppliers).test()
<Suppliers>
  <Supplier>
    <row>1</row>
  </Supplier>
  <Supplier>
    <row>2</row>
  </Supplier>
</Suppliers>

The relevant documentation is "Controlling the Form of the Projection for Collection Properties" here.

Let me add a few notes.

If a class extend Ens.Response or Ens.Request  is not necessary to extend %Persistent because is already inherited by Ens.Request/Ens.Response.
Edit: please see Antoine commnet below

If the desired XML tag correspond to the class name (without package), it is not necessary to add the XMLNAME parameter, like you did in App.Objects.Suppliers class.

Same goes for XMLNAME property parameter XMLNAME = "row" is not necessary because the property name correspond to the desired XML tag name.

Please check "Enabling Tracing" documentation here.

Optionally, instead of using  IgnoreNull property of the reader, add to your M.PAT class:

Parameter XMLIGNORENULL = 1;

Please check the documentation here regarding the IgnoreNull property of the reader.

OK, I think I understand the question! 😊

Suppose you want to put the "source" HL7 Message SourceConfigName in MSH:Security:

<assign value='##class(Ens.MessageHeader).%OpenId($$$JobCurrentHeaderId).SourceConfigName' property='target.{MSH:Security}' action='set' />

In case you what to get the first message source config name:

<assign value='##class(Ens.MessageHeader).%OpenId($$$JobSessionId).SourceConfigName' property='target.{MSH:Security}' action='set' />

To "capture" output I'd use a pipe instead of $ZF(-100), here a code sample:

Class Community.ExecOS [ Abstract ]
{

/// run an operating system commandClassMethod ExecOS(cmd As%String, Output LinesOut As%String)
{
    set oldzeof=$SYSTEM.Process.SetZEOF(0)
    set io=$IO,LinesOut=0try {
        if cmd=""Quitopen cmd:"Q"use cmd
        for LinesOut=1:1 {
            read LinesOut(LinesOut)
        }
    } catch CatchError {
        If CatchError.Name'="<ENDOFFILE>" {
            set LinesOut=CatchError.Name
        }
    }
    close cmd
    Do$SYSTEM.Process.SetZEOF(oldzeof)
    Use io
    quit
}

}

EPTEST>d ##class(Community.ExecOS).ExecOS("dir",.out) 
EPTEST>zw
out=13
out(1)=" Volume in drive C has no label."
out(2)=" Volume Serial Number is 3E11-87B5"
out(3)=""
out(4)=" Directory of c:\intersystems\irishealth\mgr\eptest"
out(5)=""
out(6)="02.01.2024  12:14    <DIR>          ."
out(7)="02.01.2024  12:14    <DIR>          .."
out(8)="11.01.2024  19:04       429'916'160 IRIS.DAT"
out(9)="05.01.2024  14:26                42 iris.lck"
out(10)="24.11.2023  00:34    <DIR>          stream"
out(11)="               2 File(s)    429'916'202 bytes"
out(12)="               3 Dir(s)  63'936'786'432 bytes free"
EPTEST>

Check ##class(HS.Util.Installer.Foundation).Install()

At the moment IRIS does not support OpenAPI 3.0, the project OpenAPI-Suite in Open Exchange ("Set of tools for ObjectScript code generation from Swagger 3.0") support it.

Check FOR SOME %ELEMENT (SQL) documentation.