Hidden fields are usually populated dynamically. They are not shown to the person filling out the form. Here's how to set the value.
Select the Field
In your form, select the hidden field and click Edit for Autofill.
If you want to capture URL Parameters (Query Strings) from the page the person is on when filling out the form, you can use URL Parameters to populate your hidden field.
Select URL Parameter for Get Value Type.
Enter the Parameter Name and click Save.
You can enter a Default Value in case the URL parameter is not found.
If you are storing data in cookies, you can use Cookie Value to pick up data when the form is submitted.
Select Cookie Value for Get Value From.
Enter the cookie Parameter Name you want and click Save.
If you want to capture data from the previous page the visitor came from before filling out the form, you can use Referrer Parameter.
Set Get Value From to Referrer Parameter.
Enter the Parameter Name which you want to snatch from the referrer URL and click Save.
Click Approve and Close.
Sweet! You're doing pretty well. There's more to learn about Forms.