How to create hidden field

Found how

I am improving this in v4.4 that will come out this week hopefully.

Great!
Thanks

pre-load hook #5 will allow you to do that, you can get the helper code from the form editor metabox

I’m afraid I didn’t understand the answer.

you need to use a hook (filter + function code) in your functions.php file.

A helper code is available on the form editor admin page in the filter metabox (see screenshot #21)