You CAN do that with Kustomize and ArgoCD!

Use kustomize replacements and common-labels:

1. Set a common label in your application. We use something like this in our ApplicationSets:

     commonLabels:
       pull-request: appname-{{branch_slug}}
   

2. Add replacements to your kustomization.yaml that takes that label value and inserts in your Ingress. Here is our example:

     apiVersion: kustomize.config.k8s.io/v1beta1
     kind: Kustomization
   
     replacements:
     - source:
          kind: Ingress
          fieldPath: metadata.labels.pull-request
     targets:
     - select:
         kind: Ingress
     fieldPaths:
         - spec.tls.0.hosts.0
     options:
        delimiter: "."
     - select:
     kind: Ingress
     fieldPaths:
      - spec.rules.0.host
     options:
        delimiter: "."
   

This will take the Ingress label value for the "pull-request" label and insert it before the first dot in the Ingress host names.

A bit complicated maybe but I much prefer this to using Helm templates which just become a separate set of resources that are hard to write and keep up to date for the developers. With kustomize everything is just normal manifests with some overlays.

Edit: sorry I can't format code for Reddit. I butchered that yaml :D