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