Asp.Net Token Signing

I am using the built-in OAuth provider in Asp.Net to generate JWT tokens - how is the token generated? I've read some places that a secret phrase is required to generate a unique key to stamp the token with - where does that get set?

This is where the token endpoint is configured in Startup.Auth.cs

public partial class Startup
    {
        public static OAuthAuthorizationServerOptions OAuthOptions { get; private set; }

        public static string PublicClientId { get; private set; }

        // For more information on configuring authentication, please visit https://go.microsoft.com/fwlink/?LinkId=301864
        public void ConfigureAuth(IAppBuilder app)
        {
            // Configure the db context and user manager to use a single instance per request
            app.CreatePerOwinContext(ApplicationDbContext.Create);
            app.CreatePerOwinContext<ApplicationUserManager>(ApplicationUserManager.Create);

            // Enable the application to use a cookie to store information for the signed in user
            // and to use a cookie to temporarily store information about a user logging in with a third party login provider

            //app.UseCookieAuthentication(new CookieAuthenticationOptions());
            // app.UseExternalSignInCookie(DefaultAuthenticationTypes.ExternalCookie);

            // Configure the application for OAuth based flow
            PublicClientId = "self";
            OAuthOptions = new OAuthAuthorizationServerOptions
            {
                TokenEndpointPath = new PathString("/Token"),
                Provider = new ApplicationOAuthProvider(PublicClientId),
                AuthorizeEndpointPath = new PathString("/api/Account/ExternalLogin"),
                AccessTokenExpireTimeSpan = TimeSpan.FromDays(14),
                // In production mode set AllowInsecureHttp = false    

            };

            // Enable the application to use bearer tokens to authenticate users
            app.UseOAuthBearerTokens(OAuthOptions);

            
        }
    }