1. Open the form document in Foxit PDF Editor
2.In File tab, choose Preferences to open Preferences dailog box.
3.Click on Forms tab in Preferences dialog box, type in the Default subject and Default contents.
4.Click on OK to apply the changes.
Relative Topics:
Designing Interactive PDF Forms From Scratch
https://www.foxitsoftware.com/blog/designing-interactive-pdf-forms-from-scratch/
Pro Tips for Creating Interactive PDF Forms
https://www.foxitsoftware.com/blog/pro-tips-for-creating-interactive-pdf-forms/