How to join public group in Telegram group based on an invite code

Someone provide this:
tg://join?invite=xxxxxxxx[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"tg://join?invite=xxxxxxxx"] options:@{} completionHandler:^(BOOL success) {}];

1. if the xxxxxxx is private invite code, it worked.
2. if the xxxxxxx is public invite code, the telegram show "This invite link has expired"

What do I do?