When embedding a form on a WordPress page or post, the code retrieved from the Publish → Platforms → WordPress section within our form builder normally looks like this:
[ 123-contact-form i507842 ]
In order to prefill form fields you need to change this code in the following way (in one line):
[ 123-contact-form i507842 control3592242-1=John&control3592242-2=Doeemail@example.com&control35 91577=This is a test message. ]
control3591576 are the name attributes of form fields. You can retrieve them by inspecting the code of your form with a browser inspection tool.
This is a test message. represent the prefilled values assigned to the fields above.
Name attributes and values are connected through the operator