Is there a possibility to use a custom (non-standard) field from an OAuth source to be mapped to the users id?

**Describe your question/**
I am trying to set up an external OAuth Provider, failing because the standard is not properly met.
We need to use this provider as we are part of a larger network and fetch user data from their database.
However, users in this network are not identified by an "id" but by a "cid".

Is there any way for me to make authentik use this field instead of the non-existent "id" field?
Can i use any workaround or is this use case not supported by authentik?

**Relevant info**
The "OAuth" provider features authorize, token and userdata endpoints, but nothing else. The cid returned by the userdata endpoint is found in an object that looks like this:

```
{
data: {
...
cid: string
...
}
}
```

**Screenshots**

This is the error that is being shown

**Logs**
I was unable to get any logs, even through using policies, as it appears that this check comes before any of the validations takes place, so i don't really see how to even remap the field before the get_user_id function is being called.

**Version and Deployment (please complete the following information):**

- authentik version: 2025.4.1
- Deployment: docker compose

**Additional context**
I found the error is being thrown in the oauth/views/callback.py OAuthCallback class, and as far as i can see (i'm no python programmer) it appears that the callback statically checks for the 'id' field, but i don't see any way to inject the id before it actually does that check.

Happy to share any helpful information!