Wizard.cs

gistfile1.cs

public class WizardViewModel
{
    public StepOne StepOne
    {
        get;
        set;
    }

    //
    // ... other steps saved for brevity
    //
}

StepOne.cs

public class StepOne
{

    [Remote("Tenant", "Validation", "Api", "host", ErrorMessage = "The provided host already exists.")]
    public string Host
    {
        get;
        set;
    }
   
    //
    // ... other properties saved for brevity
    //
}

ValidationController.cs

public class ValidationController : Controller
{
    private readonly ITenantService _tenantService;

    public ValidationController(ITenantService tenantService)
    {
        _tenantService = tenantService;
    }

    public ActionResult Tenant(string host)
    {
        var tenant = _tenantService.FindByHost(host);

        return Json(tenant == null, JsonRequestBehavior.AllowGet);
    }
}