Custom Fields in Forms
You can collect custom information about a contact using custom fields.
As an example, add an "input" field in the form builder.
Clicking the button will bring up the field UI where you can give your field a name (circled). This name is how you will reference the field moving forward with replacement codes and in the Custom Info tab.
When a contact fills out the form, the field will show in their contact record under the custom info tab.