Unable to install plugin in external Grafana using url for helm-installed Grafana OnCall ( 403 - Enable Javascript and cookies to continue )

What went wrong?

What happened:

I followed the instructions for installing Grafana OnCall using helm
It's up and running under a CNAME in my company's domain (e.g. oncall.mycompany.com)
Our grafana is running on a different domain (e.g. grafana.services.otherdomain.com)

When attempting to install the Grafana OnCall Plugin and pointing at https://oncall.mycompany.com the install fails with

An unknown error occurred when trying to install the plugin. Verify OnCall API URL, https://oncall.mycompany.com, is correct?
Refresh your page and try again, or try removing your plugin configuration and reconfiguring.

And the browser network log shows a 403 when trying to hit https://grafana.services.otherdomain.com/api/plugin-proxy/grafana-oncall-app/api/internal/v1/plugin/self-hosted/install. The 403 response is an HTML page with the text "Enable Javascript and cookies to continue"
```
Failed to load resource: the server responded with a status of 403 ()
Header.styles.ts:6 An HTTP related error occurred while trying to install the plugin w/ the OnCall backend Object
```
What did you expect to happen:
The install of the Plugin to succeed

How do we reproduce it?

Install Grafana OnCall Helm Chart for external grafana and external ingress configured to create ALB with mycompany.com cert
Create a CNAME entry for the ALB (oncall.mycompany.com)
Open grafana and attempt to install the OnCall Plugin pointing it the CNAME created in step 2 (i.e. https://oncall.mycompany.com)

Grafana OnCall Version

v1.8.3

Product Area

Helm/Kubernetes/Docker

Grafana OnCall Platform?

Kubernetes

User's Browser?

Both Chrome and Safari

Anything else to add?

No response

grafana / oncall

Unable to install plugin in external Grafana using url for helm-installed Grafana OnCall ( 403 - Enable Javascript and cookies to continue ) #4699