You can pre-fill any input on Action Network pages or widgets by using URL arguments corresponding to the name of the input. For example, if you wanted to pre-fill a user's first name on a petition, you'd use the URL argument
signature[first_name], like so:
You can use arguments to fill any input on the page, hidden or not. You'll want to look at the generated HTML to determine the field's name. For instance, the first name field for forms is different from petitions.
Note: Be careful with this -- you can fill hidden inputs and inputs we use to make actions work. Make sure you understand how inputs work on actions (both with recognized and non-recognized users) and make sure your input names don't collide with ones we use accidentally.