You can use the 123 Form Builder – NationBuilder integration to associate your form fields with custom fields created in your NationBuilder account. To create a custom field, log into the dashboard of your NationBuilder account, head over to the Settings → Nation Defaults section, and click on Custom Fields.
Note: The only field types you can create are text, number, checkboxes, and multiple-choice fields. For the integration to work properly, your form fields need to have the same field validations as your custom fields. This means that fields such as the dropdown list are not compatible with custom fields from NationBuilder.
In your 123 Form Builder account, go to the Applications section of your form and hit the Customize button of the 123 Form Builder – NationBuilder app. Select the form field you want to associate with the custom field in NationBuilder and click on the dropdown list on the right. Scroll down until you see the Custom Fields group, where all the NationBuilder custom fields will be displayed below. Associate the custom field you want, save changes, and you are done.