If your site has a password-protected area and its users are connected to Salesforce Contact records in your Salesforce instance, you can use the value of any field on the Contact object to prepopulate the fields on a Forms page.
To do so, insert the Salesforce Field Name as the Default Value in the following format:
For custom fields, the Field Name will always contain "__c":
Locating Field Names in Salesforce
To locate the Field Names for your Contact object in Salesforce, go to Setup > Customize > Contacts > Field Names. The Field Name is identified in the middle column of this view:
If you are viewing the Contact object, you can use the blue tab with the arrow on the right of the page to easily access the Setup view in Salesforce. After clicking the arrow, a panel will expand. Click View Fields.
Locating Field Names in Soapbox
The Field Name for Salesforce is shown in brackets for a Salesforce Element. if you've included the Contact field you wish to use as the Default Value for a form, you can see that Field Name in Soapbox.
Example Use Case: Form posting to Lead Object
If you have a form posting to the Lead object for a petition, a contact us form, or some expression of interest, you can use the variables from the logged in user's Contact record to populate corresponding variables on the Forms page posting to the Lead object so the logged in user doesn't need to enter those fields manually.
For a list of all variables available in Forms, refer to Variables available for Forms default values. For details on using the logged in user's Soapbox username or email address as default values, go refer Prepopulating values with logged in user's Username and Email. For information on using the visitor's IP Address, refer to Using submitter's IP address as a Default Value.
For information on how to use prepopulated values to create records in Salesforce that are children of the logged in user's Account or Contact, refer to Creating records related to logged in user's Contact or Account record.