There are several .Net sample codes that using oauth2 to signin Xero, but all of them is working with browser (most of the sample code for C# is for ASP.NET MVC).
I am going to develop a desktop application using Xero Api but Xero official tutorial said that mobile or desktop application should use PKCE flow instead of using ClientID/Secret scheme.
And in this PKCE flow, user should interact with browser to get Authorization code.
I want to know if there is any way to signin without user interaction.
Xero api signin was just migrated to use OAuth2.0 and it should interact with browser at least once. Once you get refresh_token using offline_access scope, you will use this token to get access token. Unfortunately they don't support custom url scheme. Hope this will be helpful for you.