Onion Architecture with .NET Core 2

Hi Devs,

I implement Onion Architecture using this link:

https://social.technet.microsoft.com/wiki/contents/articles/36655.onion-architecture-in-asp-net-core-mvc.aspx

Until now everything is ok, the example is implemented using the repository pattern(only crud), but lets say that for my User Domain I need to implement a custom method like search user by Username, how can I implement this custom method using onion architecture used in this example?

Best Regards