if your client_id is passed as a form param, you‘ll have to get it by doing context.TryGetFormCredentials(out clientId, out clientSecret);
if your client_id is passed as an Authorization header, you can get it by doing context.TryGetBasicCredentials(out clientId, out clientSecret);
once you‘ve got the client_id from your request, do context.Validated(clientId), this will set your context.ClientId property, this property will always be null until you‘ve done context.Validated()