I've been trying to get OAuth 2 with the PKCE flow going, following the documentation at https://developer.xero.com/documentation/guides/oauth2/pkce-flow/

I've not been able to get that to work, kept on getting a 400 "invalid_client" response on the step to exchange the verification code for an access token (the POST to https://identity.xero.com/connect/token).

I tried also with the code flow shown on https://developer.xero.com/documentation/guides/oauth2/auth-flow/ and eventually did get that to work, but had to deviate from the guidance.

The guidance there says to use header of:
Authorization: "Basic " + base64encode(client_id + ":" + client_secret)

But I think that's wrong. I downloaded the working sample from Github (https://github.com/XeroAPI/xero-netstandard-oauth2-starter-app-dotnet-framework) and used Fiddler to examine the POST and it does not use an Authorization header at all - and so when I modified my non-working code to not use the Authorization header and instead put the client_id and client_secret in the body it worked.

That sample project uses the Xero.NetStandard.OAuth2.Client.XeroClient so perhaps things have changed a little and the documentation hasn't kept up.

So anyway, I'm thinking the guidance around PKCE flow might also be a little out of date also - is there a working example anywhere of using PKCE?

I'm wanting to integrate into a desktop application, but I can't find any suitable examples.

Thanks!