Is there a way to set a minimum characters length in the itemform? I have found that html5 has this option:

I would like to do this, but I don't know how.

Sorry about my english and thanks.

Hello if you want this on the backend you need to add via a K2 plugin (you can add it with a simple JS script).

On the frontend you can override the itemform.php file.
This post will help you with the overrides getk2.org/documentation/tutorials/174