When embedding a form on a WordPress page or post, the code retrieved from the Publish → Platforms → 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):
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: