When embedding a form on a WordPress page or post, the code retrieved from the Get Code & Publish → WordPress section normally looks like this:
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=Doefirstname.lastname@example.org&control35 91577=This is a test message.]
control3591576are the name attributes of form fields. They can be retrieved by inspecting the code of your form with a browser inspection tool.
This is a test message. are the prefilled values assigned to the fields above.
Name attributes and value are connected through the operator
This example has the following result:
[123-contact-form i507842 control3592242-1=John&control3592242-2=Doeemail@example.com&control3591577=This is a test message.]