How to get a ResourceReference from a Resource?

[johnkwaters]

In the example below, I first fetch a Patient.
I then want to add an Encounter, with the Patient as the Subject, and then an Observation, which has the Patient as Subject, and references the Encounter.

var pu = await fc.ReadAsync<Patient>(
    ResourceIdentity.Build("Patient", "ORG-SM-05306f3b-c7c3-4f9a-a441-87feb2ec6a66"));
pu.Address.First().PostalCode = "01008";

var encounter = new Encounter
{
    Period = new Period(new FhirDateTime(2022, 5, 15), new FhirDateTime(2022, 5, 15)),
    Status = Encounter.EncounterStatus.Finished,
};

var eid = new Identifier(HC_SYSTEM, Guid.NewGuid().ToString());
encounter.Identifier.Add(eid);
encounter.Subject = pu;

var observation = new Observation
{
    Status = ObservationStatus.Final,
    Code = new CodeableConcept(CodeSystemConstants.LOINC, LOINC.BODY_WEIGHT),
    Value = new Quantity((int)(310/2.2), "kg"),
    Encounter = encounter,
    Subject = pu
};

var updated = await fc.UpdateAsync(pu);

My problem is I can't do what I am doing above - as "pu" and "encounter" are resources, and I need to get their resource references. I would assume there is some help method that can get that reference from a Resource?

And is there a better way to do this with TransactionBuilder?