When creating a custom Sitecore Experience Form Field that needs to be mapped to a Salesforce field in the S4S Form Field Mapping Wizard, it is necessary to add the custom form field Template ID's seperated IDs separated by commas to the CustomFormFieldTemplateIds setting in the App_Config\Modules\S4S\S4SMappings.config file.
The Field Template for is referenced in the ‘Field Template’ field on the custom field:
...
Template Ids for built-in field types not included by default can be added in the same way. In this case, the ID can usually be found under Templates\System\Forms\Fields (shown selected below):
...