You may want to make a data field "hidden" in a form so you can track data that is important, but not important to the person filling out the form. 

An example of this is hiding a "Lead-Source" field. You may want to know where the lead source came from, yet you don't want the subscriber to have to fill this information out.

After you have started the process to create a website form, you will find yourself on a page that allows you edit your form details.

Hover over the title of the data field name with your mouse and a pencil icon will pop up - click on it.

A popup box will appear. Click the box next to "Make field hidden" and type in your default value. The default value is what will show up in when the contact information is added/updated in your contact list. The person filling out the form will not see it. When you're done, click the "Done Editing" button.

Follow the next steps to complete the process to create a form.