Hidden fields are usually populated dynamically. They are not shown to the form-filler-outer (That's a technical term, feel free to use it). Here is how to set the value:
Go to the Marketing Activities area.
Select your form and click Edit Form.
Select your hidden field and click Edit for Autofill.
Use Default Value
By selecting Use Default Value you can hard-code a specific value to always be used when this form is submitted.
Enter the Default Value and click SAVE.
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.
If you are storing data in cookies, you can use Cookie Value to pickup 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 clickSAVE.
Click APPROVE AND CLOSE.
Sweet! You're doing pretty good. There is more to learn about Forms.