Kafka Go/Sarama and schema registry registration issues with protobuf

Question

I have a compiled proto from another repo that I would like to have registered for an integration test.

I'm basically doing this:

    payload := map[string]interface{}{
        "schemaType": "PROTOBUF",
        "schema":     "",
    }
    payloadBytes, err := json.Marshal(payload)
    if err != nil {
        return -1, err
    }

    subjectWithValue := fmt.Sprintf("%s-value", subject)
    resp, err := http.Post(fmt.Sprintf("%s/subjects/%s/versions", s.Cfg.RegistryAddress, subjectWithValue), "application/vnd.schemaregistry.v1+json", bytes.NewReader(payloadBytes))
    if err != nil {
        return -1, err
    }
    defer resp.Body.Close()

    if resp.StatusCode != http.StatusOK {
        body, _ := io.ReadAll(resp.Body)
        return -1, errors.Errorf("Failed to register schema, status: %d, response: %s\n", resp.StatusCode, string(body))
    }

    var response map[string]interface{}
    if err := json.NewDecoder(resp.Body).Decode(&response); err != nil {
        return -1, err
    }

    schemaID := int(response["id"].(float64))
    s.Logger.Info("Registered schema with ID", zap.Int("schema_id", schemaID))
    return schemaID, nil

but I don't know how to get the schema on line 3.

I've tried importing and deriving like so:

    protoItem := &myPackage.MyProtoCompiledInGo{}
    schema := string(protoItem.ProtoReflect().Descriptor())

I have integration tests where I produce and consume records using this proto. However, It seems to be failing, presumably because the schema isn't registered?