You do have a Premium X account but you don't see this checkmark just below the composer?
Here are the steps on how to enable this option:
Close the TweetHunter tab on your browser;
Log out of your Premium X account or any other X accounts you have opened at the moment;
We would suggest to clear the cache from the browser and relaunch it (not mandatory);
Log back into your Premium X account;
Please wait for up to 24 hours (should appear sooner) for TweetHunter to register the Long form posts option on your account and display it for you.
If you are an author on X, or are a bit more seriously involved with X, we would highly suggest this upgrade, as the character limit goes from 280 characters for regular users to 25,000 characters for Premium X users, which is a significant increase.
In the event that the option is not showed in your composer 24 hours after you completing the process, please reach out to our Customer Support, either via chat or via email, and we will make sure to assist you as quickly as possible!