There are probably a thousand WordPress form generators out there, with old ones disappearing and new ones appearing faster than anyone can keep track. Some of them deal with script-inserted fields gracefully, and some do not.
Where to add the TrustedForm script to your form page.
Our customers have some success by placing the TrustedForm script either in the same place allotted in WordPress for Google's SEO tool, and also some success placing it using the WordPress Headers and Footers Scripts tool.
These screen shots illustrate where to find those options in your WordPress dashboard.
WordPress for Google's SEO tool:
WordPress Headers and Footers Scripts tool:
Don't include a TrustedForm field in your form.
You should not, in general, create a TrustedForm hidden field in your form using your form generator. The TrustedForm script will add the necessary hidden field to the form in the user's browser memory at run time. Any same-named field already in place could interfere with that process.
Checking whether the script is working.
You can check a loaded form page for the presence of a populated hidden field named "xxTrustedFormCertUrl_0" in the browser's page DOM by using the browser's "inspect" feature. (It will not be visible in the "view page source" feature because it exists only in memory, not in the source code.)
Will your form generator capture and submit the TrustedForm Cert URL field?
There's still no guarantee that the field added by the TrustedForm script will be processed and submitted. That depends on how any scripts that are products of the form generator interact with the form.