The hidden field is a field that is not visible on the form outside of the form builder, and as such can be used to include information in form submissions that you may not want seen by those who are submitting it.
Using Hidden Fields
The field functions similarly to a standard text field, except instead of mapping whatever the form submitter types into the field into CRM, it maps whatever text you set as the default value of the field in the field properties. This can be really useful for things such as using the form to set the lead source field value.
What makes the hidden field special (other than the fact that it is invisible), and what many users may not realize, is that it can be mapped to both text and picklist fields in CRM.
The mapping section of the hidden form field itself looks like the mapping section of a text field and only provides the option to map to text fields on the lead and contact records.
However, if you add the hidden field to a form in the form builder, access its properties, and open the Mapping tab, you'll see the option to map the field to either a text field or a picklist field.
If you want to map the hidden field to a picklist field, you'll have to be sure to set the default value in the hidden field's properties as the value of the desired picklist response rather than the label. And check the Override in CRM box in order for the Lead Source field to always update the lead record.
Where to Find the Lead Source Value
You can find the corresponding values for the options in the Lead Source picklist field by navigating to Settings > Customizations > Customize the System > Entities > Lead (or Contact) > Fields, and opening the record for the picklist field. The value can be text or a number. If it is a large number with commas in it, such as 100,000,001, ignore the commas and only use the numbers as the value (100000001).
Once you know the correct value, go back to your form builder and place that value into the Default Value field of the hidden field's properties. Now when the form is submitted, the lead source field will be set to "Forward to a Friend referral."
That's all there is to it! Now whenever somebody submits your form, their Lead Source will automatically be set to Internet Marketing.