Based on the current way that recipe is written (I have an update pending), you’ll take the contents of the upstream values.yaml file, and paste it (indented 4 spaces) into your configmap, then make any necessary changes.
So you might end up with something like this:
apiVersion: v1
kind: ConfigMap
metadata:
name: metallb-helm-chart-value-overrides
namespace: metallb-system
data:
values.yaml: |-
# Default values for metallb.
# This is a YAML-formatted file.
# Declare variables to be passed into your templates.
imagePullSecrets: []
nameOverride: ""
fullnameOverride: ""
<continued...>