ASP.NET CORE Sign In Cookie

Question

I am having a lot of trouble understanding some things in Asp.NET Core. I already have a Asp.NET 4.5 application that has login authentication using FormAuthenticationTicket but my goal is to set up a Core Web Api that authenticates a user and creates a cookie for my 4.5 Application to read, and on redirect to already be signed in via cookie.

I have given both applications the same <machinekey> in the web.config and added UseCookieAuthentication with CookieAuthenticationOptions to Startup.cs but I am at a loss from here on how to replicate the FormsAuthenticationTicket inside my ApplicationController.cs in my Core application. I find that the documentation for Core is not overly consistant yet but I have been trying out a lot of suggestions to no avail.

I think the main confusion for me is that I can create a cookie in Core I am clearly not creating it correctly or most likely not authenticating correctly either.

Startup.cs in Configure function

app.UseCookieAuthentication(new CookieAuthenticationOptions
{
     AuthenticationScheme = "ApiAuth",
     CookieName = ".ASPXAUTH",
     CookieHttpOnly = false,
     ExpireTimeSpan = TimeSpan.FromDays(30),
     SlidingExpiration = true,
     AutomaticAuthenticate = true,
     LoginPath = new PathString("/Application/Authorize"),
});

ApplicationController.cs

[HttpGet("Authorize/{appGuid}/{userGuid}", Name = "SignIn")]
public async Task<IActionResult> SignIn(Guid appGuid, Guid userGuid)
{
     var application = Application.Find(appGuid);
     var user = User.Find(userGuid);

     if (application != null && user != null)
     {
          await HttpContext.Authentication.SignOutAsync("ApiAuth");

          /****************Confusion start****************/

          Claim cookiePath = new Claim(ClaimTypes.CookiePath, ".ASPXAUTH");
          Claim expiration = new Claim(ClaimTypes.Expiration, DateTime.UtcNow.AddDays(30).ToString());
          Claim expiryDate = new Claim(ClaimTypes.Expired, "false");
          Claim persistant = new Claim(ClaimTypes.IsPersistent, "true");
          Claim issueDate = new Claim("IssueDate", DateTime.UtcNow.ToString());
          Claim name = new Claim(ClaimTypes.Name, user.Username);
          Claim userData = new Claim(ClaimTypes.UserData, "");
          Claim version = new Claim(ClaimTypes.Version, "2");
          ClaimsPrincipal principal = new ClaimsPrincipal(new ClaimsIdentity(new[] { cookiePath, expiration, expiryDate,
                    persistant, issueDate, name, userData, version }, "ApiAuth"));
          await HttpContext.Authentication.SignInAsync("ApiAuth", principal);

          /****************Confusion end****************/

          return new RedirectResult("http://localhost/MyWebsite/Repository.aspx");
     }

     return Unauthorized();
}

The size of the cookie is much larger than the one on my 4.5 application and I am at a loss as to where to go from here. I believe I am also causing conflicting settings with UseCookieAuthentication and the ClaimsPrincipal.