Updating View with changed ViewModel after Form Post - asp.net-mvc

Question

I'm trying to update an MVC view to display a message after a form has been posted but cannot get it working. I'm new to this so would appreciate a few pointers on what I'm doing wrong. This isn't form validation as the logic and response message won't be known until after the form has been posted and processed.
The controller action [HttpPost] public async Task<IActionResult> PlatformRecord(RecAction RecActionNotUSed) fires on the Form POST and does some codebehind logic - I then need to update the View with a Response which is only a temporary message and not stored anywhere.
The initial GET request works fine and does display the message but just cannot figure out how to do the same after a Form POST.
I've tried adding ModelState.Clear(); without success. Also, if I redirect to the initial view I lose the Response message eg return RedirectToAction("PlatformRecord"); means I no longer have RecAction.Response = "NEED TO SHOW THIS RESPOSNE MESSAGE AFTER FORM POST.";
My code is as follows
VIEWMODEL:
public class RecAction
{
public RecUser RecUser { get; set; }
public string Response { get; set; }
}
CONTROLLER:
public class RecordManagerController : Controller
{
private readonly IOptions<ConnectionStrings> _connectionStrings;
private readonly UserManager<AppRecationUser> _userManager;
Public RecordManagerController(UserManager <AppRecationUser> UserManager,
IOptions <connectionStrings> connectionStrings)
{
_userManager = userManager;
_connectionStrings = connectionStrings;
}
// GET: /<controller>/
public IActionResult Index()
{
return View();
}
private Task<AppRecationUser> GetCurrentUserAsync()
{
return _userManager.GetUserAsync(HttpContext.User);
}
public async Task<IActionResult> PlatformRecord()
{
var RecordDataModel = new RecordDataModel(_connectionStrings.Value.DefaultConnection);
var user = await GetCurrentUserAsync();
RecAction RecAction = new RecAction();
RecAction.RecUser = RecordDataModel.GetRecord(user.Email, "Platform");
if (RecAction.RecUser.Record == null)
{
//Response Successfully Displayed
RecAction.Response = "No Record found";
}
return View(RecAction);
}
[HttpPost]
public async Task<IActionResult> PlatformRecord(RecAction RecActionNotUSed)
{
try
{
if (ModelState.IsValid)
{
var RecordDataModel = new RecordDataModel(_connectionStrings.Value.DefaultConnection);
var user = await GetCurrentUserAsync();
RecAction RecAction = new RecAction();
RecAction.RecUser = RecordDataModel.GetRecord(user.Email, "Platform");
RecSettings latestSettings = RecordDataModel.GetSettings();
RecKeys RecKeys = RecordDataModel.GetKey();
if (RecKeys.PrivateKey == null)
{
ModelState.Clear();
//Rsponse not updating
RecAction.Response = "NEED TO SHOW THIS RESPOSNE MESSAGE AFTER FORM POST.";
return View(RecAction);
}
return RedirectToAction("PlatformRecord");
}
Else
{
//Need to return the same view for errors so the validation is not overwritten.
return View();
}
}
catch
{
// If we got this far, something failed, redisplay form
return RedirectToAction("PlatformRecord");
}
}
}
VIEW:
#model ProjectXYZ.Models.RecordModels.RecAction
#{
ViewData["Title"] = "PlatformRecord";
}
<h2>Platform Record</h2>
<form asp-controller="RecordManager" asp-action="PlatformRecord" asp-route-returnurl="#ViewData["ReturnUrl"]" method="post" class="form-horizontal" autocomplete="off">
<p></p>
<hr />
<div class="form-group">
<label asp-for="RecUser.Record" class="col-md-2 control-label"></label>
<div class="col-md-10">
<textarea asp-for="RecUser.Record" class="form-control" cols="1" rows="8" readonly></textarea>
</div>
</div>
<div class="form-group">
<label class="col-md-2 control-label"></label>
<div class="col-md-10">
<input asp-for="Response" class="text-danger" readonly/>
</div>
</div>
<div class="form-group">
<div class="col-md-offset-2 col-md-10">
<button type="submit" class="btn btn-default">Request New Record</button>
</div>
</div>
</form>