I have finally published and open sourced my libraries (MIT). You can find it here https://github.com/codecapital/CodeCapital.Bullhorn
Also, the library is available as a nuget package here https://www.nuget.org/packages/CodeCapital.Bullhorn
The project contains Bullhorn library (3 years in development) which can be referenced from a .NET 5 console app or any .NET 5 website.
Also, you can use Bullhorn.CommandLine as a playground to test your queries or authentication.
There is not any documentation at the moment so this library is recommended if you are familiar with C#.
I am planning lots of refactoring so I might write also some documentation.
This library is using native System.Text.Json for deserialization which is more powerful than traditional Newtonsoft.Json, also Microsoft.Extensions.Http.Polly is used for more resilient connection and inbuilt retry policies. IdentityModel is used for authentication but Bullhorn authentication is clumsy so an extra code was needed.
P.S. I am planning to release also Bullhorn.Query which is using the library above to query Bullhorn REST API through very basic UI. This is purely for a developer testing purpose.
I hope this will help some developers here.