New action 'call' in Form plugin

Hello all!
I’m newbie in grav and php (and english)). Where i must put my function or class so that new Form action ‘call’ can run it?
I put public static function in form.php in Form plugin directory but i got ‘Crikey!..’ error in browser when click submit button.

If you are creating a custom form action, you will need to create a new plugin to add the actual php code. See next docs:

dont modify core plugins, your changes will be lost on update anyway.

Thanks, hugoaf!
But i’m about new action ‘call’ in core plugin Form, you can see it in the #407 line of code: