public abstract class ViewModelBase

protected abstract void RegisterCommands();

protected override void RegisterCommands()
{
    LoginCommand?
        .Configure(
            execute: (msg) => { Login(User); },
            canExecute: (x) => { return CanLogin(); }
            );
}

protected abstract void RegisterCommands();

public ViewModelBase()
{
    RegisterCommands();
}

public class LoginViewModel : ViewModelBase
{

    private ICustomCommand _loginCommand;

    public ICustomCommand LoginCommand
    {
        get
        {
            return _loginCommand;
        }
        set
        {
            _loginCommand = value;
        }
    }

    public LoginViewModel(ICustomCommand loginCommand)
    {
        _loginCommand = loginCommand;
    }

    protected override void RegisterCommands()
    {
        LoginCommand?
            .Configure(
                execute: (msg) => { Login(User); },
                canExecute: (x) => { return CanLogin(); }
                );
    }

public abstract class ViewModelBase
{
    public ViewModelBase(ICustomCommand loginCommand)
    {
        _loginCommand = loginCommand;
        RegisterCommands();
    }
}

public LoginViewModel(ICustomCommand loginCommand):base(loginCommand) { }