We are doing something similar using Joomla. While the way to do it would be different, the concept is the same. We add to the link from Joomla a querystring value to Forms so that fields are pre-populated. You would need to find out how to pass the user value you want from WordPress into the querystring, but the rest of it should work after that. Here is information about how the querystring works in Forms:

Q: Is it possible to pre-fill the fields on a form by passing in URL parameters?
A: Yes, it is! You can add variable parameters onto the URL to a form. In order to do so, you'll need to know the variable name associated with the field you want to fill in, and have the link to that form. Once you have this information, build the URL with the following structure:

base URL?variable1=value&variable2=value

Replace variable1 and variable2 with the variable names of the fields you want to populate on the form, and replace value with the value you want to appear in those fields. Append other variables to the URL by inserting an & symbol followed by the variable name, as shown above.

Note: Only the Single Line, Multi-line, Number, and Currency fields are supported with this feature.