This functionality is currently being revised. See in-app documentation.
In your git provider, GitLab for instance, create a Deployment Token...

Copy the data from your git provider into the Container Registry Secrets fields

Save this data to generate the Secret key

Copy this key - we'll use it in the HPA Deployment HELM Chart
In the HELM Chart, paste this key under Image Pull Secrets...

This lets the HELM Chart connect to your Container Registry to pull images.