In the section "Script called before form is generated" I want to use the id of the form. What is the code line for this?

See this link

If you create a hidden field 'formid' then set its default value to {global:formid} you can access it in the script area with

Thank you for this tip.
But in this case the $val isn't filled yet.

I carried out a little test by creating a text box 'formid' with the {global:formid} as before and putting the following into 'scripts processed before generation' it does output 'changed it' instead of the forms actual id. That suggests the formid global exists from the moment you create the form

I've created a hidden field with the value "{global:formid}". When the form is loaded the value is indeed the form id. But $val at "Script called before form is generated" is still empty.
What do you see when you place this code under "Script called before form is generated"

echo '<pre>';
print_r($val);
exit;

At this point I want to know the form ID because a custom script needs it.
When the rest of the form loads your solution works.

Just an empty array()

Indeed. So the same problem as I have
So how do you know the form ID at this point? Now take it from the non-SEF url, but this doesn't work when the form is used in a module. You can check for both, but that's extra lines of code. Maybe there is a much quicker way?

Not that I know, sorry I couldn't help. Start a ticket they may have a simple answer for you.