Custom JSON structure in .NET Core

Question

I'm trying to learn .NET Core; my background is largely PHP where the absence of a type system makes what I'm trying to do here trivial.

I'm trying to create an API with a small wrapper for the response. So that instead of sending {"user": "..."} I can send {"message": "...", "data": {"user": "..."}}

To start, I began with a POCO for the wrapper:

public class AppResponse<T>
{
    public string Message { get; set; }
    public T data { get; set; }
}

Then, I instantiate the wrapper differently each time I need a response object:

[HttpPost]
public async Task<IActionResult> Register(RegisterViewModel model)
{
    if (ModelState.IsValid)
    {
        var user = new ApplicationUser { UserName = model.Email, Email = model.Email };
        var result = await _userManager.CreateAsync(user, model.Password);

        if (result.Succeeded)
        {
            var okResponse = new AppResponse<RegisterViewModel>();
            okResponse.Message = "Your account has been created.";
            return Ok(okResponse);
        }
        else
        {
            var errorResponse = new AppResponse<IEnumerable<IdentityError>>();
            errorResponse.data = result.Errors;
            return BadRequest(errorResponse);
        }
    }
    var badRequest = new AppResponse<RegisterViewModel>();
    badRequest.data = model;

    return BadRequest(badRequest);
}

Is there an easier/better way to do this in .NET Core? I intend for this to be a Web API, so the ability to make more complicated JSON responses (for things such as pagination, validation errors, etc.) is crucial.

I apologize for the confusion of camel case in variables, I'm still figuring out best practices in C#.

EDIT: I have simplified my code to use a more RESTful approach. This is it currently:

public async Task<IActionResult> Register(RegisterViewModel model)
{
    if (ModelState.IsValid)
    {
        var user = new ApplicationUser { UserName = model.Email, Email = model.Email };
        var result = await _userManager.CreateAsync(user, model.Password);

        if (result.Succeeded)
        {
            return Ok(user);
        }
        return BadRequest(result);
    }

    // If we got this far, something failed.
    return BadRequest(new
    {
        Errors = ModelState
    });
}