How to integrate external apps (outside yunohost) like HabitSync for OIDC

[protonaut]

I use dex within yunohost for managing the access for apps like headscale, outline and headplane.
Their configuration is managed during installing the apps within yunohost.
I discovered that the config ist located in /var/www/dex/config.yaml.d
For example headscale has a file /var/www/dex/config.yaml.d/headscale with the following content:

- id: headscale
  redirectURIs:
  - https://tailscale.domain1.de/oidc/callback
  name: headscale
  secret: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Now I want to use dex for handling the OIDC for an extern appliction, in particular HabitSync which is not in yunohost's application pool.
My first approach was to contact the developer of HabitSync on github:
Here is the Link

It turns out I have to additionally config some cors config of dex. But I can't find any wiki or HowTo to get to know how to config dex in that way.
I "essentially need to tell dex that it should add these cors headers so that the browser does not block the request to fetch the token from dex."

Could anybody help me with that or give me a hint where to find the answer?