MCreator issue tracker has been migrated to GitHub.
Existing tickets have been migrated to GitHub. Please open new tickets on the GitHub tracker instead. If you do not have a GitHub account, all feature requests and bug reports from forums are still considered and checked by the maintainers.
Check the tutorial on this page: https://mcreator.net/news/42657/mcreator-tutorial-using-procedures-cust…
Part of it shows how to make GUI and attach it to block and make it open.
If you still need help, search on the forums as there is plenty of discussion on forums on the procedure system already.