ASP.NET Core authenticating with Azure Active Directory and persisting custom Claims across requests

I have a default ASP.NET Core website created within Visual Studio 2017. I have chosen to authenticate using an Azure Active Directory. I run the site and can successfully login using an account in the Active Directory.

I can retrieve Claim information provided by Active Directory, e.g. by calling the following line I get the 'name'.

User.Claims.FirstOrDefault(c => c.Type == "name")?.Value;

I want to add a custom claim - CompanyId = 123456 for the logged in user. I'm able to add a custom claim however it is only available on the page where the claim is set.

Claim claim = new Claim("CompanyId", "123456", ClaimValueTypes.String);
((ClaimsIdentity)User.Identity).AddClaim(claim);

My understanding is that I somehow need to update the token that has been issued by Active Directory or set the claim before the token is issued. I'm unsure how to do this.

I suspect this needs to be done in the AccountController at SignIn()

// GET: /Account/SignIn
[HttpGet]
public IActionResult SignIn()
{
    return Challenge(
            new AuthenticationProperties { RedirectUri = "/" }, OpenIdConnectDefaults.AuthenticationScheme);
}

I've read numerous articles and samples about this scenario (including https://github.com/ahelland/AADGuide-CodeSamples/tree/master/ClaimsWebApp) however have not managed to solve how to persist the Claim across requests.

I have successfully managed to persist custom Claims using ASP.NET Identity as the Authentication Provider, but this appears to be because the custom Claim is saved to the database..