Hidden fields are usually populated dynamically. They are not shown to the person filling out the form. Here's how to set the value:

Edit Form

Go to the Marketing Activities area.

 

Select your form and click Edit Form.

Select the field you want to hide, click the Field Type drop-down and select Hidden.

With the hidden field still selected, 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.

URL Parameter

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.

Parameters are kinda techie, aren't they? Once you get them though, they are powerful. This Wikipedia page on Query Strings is somewhat helpful.

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.

Cookie Value

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.

You can enter a Default Value in case the parameter/cookie is not found.

Referrer Parameter

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.

You can enter a Default Value in case the referrer parameter is not found.

Click Finish.

Click Approve and Close.

 

Sweet! You're doing pretty well. There's more to learn about Forms.