public override async Task GrantResourceOwnerCredentials(OAuthGrantResourceOwnerCredentialsContext context)
{
    using (UserManager<IdentityUser> userManager = userManagerFactory())
    {
        IdentityUser user = await userManager.FindAsync(context.UserName, context.Password);

        if (user == null)
        {
            context.SetError("invalid_grant", "The user name or password is incorrect.");
            return;
        }

        ClaimsIdentity oAuthIdentity = await userManager.CreateIdentityAsync(user, context.Options.AuthenticationType);

        ...

        var ticket = new AuthenticationTicket(oAuthIdentity, properties);

        context.Validated(ticket);
        context.Request.Context.Authentication.SignIn(oAuthIdentity);
    }
}

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