Login.register with math hook - howto ?

Hi,

I need clarification about the math hook.
I'm using it on a classic contact form driven by Formit. No pb.

I want to add it to a registration form the same way.
the Login snippet, including the register one seams to include a math prehook. (line 480 of Loginhooks.class.php)

I added the captcha chunk from my contact form to the registration form. It appears of course but without any numbers


I guees this is because the placeholders "fi.xxx" can't be there (extract from the $captcha-culturekey) :


So how to use math hook with a register form ?

thank you