Hi @alexnemes,
you can make http requests inside the credentials files using the authenticate method.
The authenticate function will be executed each time your descriptive node makes an api request. You have access to the credentials and you can manipulate the requestOptions, i. e. setting headers or query parameters. Since personio seems to require requesting an access token before each api request, you could also do this inside the authenticate function.
Here are some n8n credential examples using the authenticate function:
That being said I am not sure if this will enable you to workaround PKCE support. We will soon continue our PKCE development but I can’t make any promises since the developer was out of office for a longer period of time.