How to use AbpSession?

I have already answered your question in comment. You have to declare a member of type IAbpSession and initialise it with the value in the constructor. You can then use the UserId, TenantId..... properties accordingly.

public class MyClass : ITransientDependency
{
    public IAbpSession AbpSession { get; set; }

    public MyClass()
    {
        AbpSession = NullAbpSession.Instance;
    }

    public void MyMethod()
    {
        var currentUserId = AbpSession.UserId;
        //...
    }
}

You can refer the documentation here.