Any Custom Field you make in your account can be used as a Smart Field when building a form.
Note: To use a Custom Field as a Smart Field, you will first have to create the Custom Field. Here's how: How to Create Custom Fields
Read on to learn how to select your existing Custom Field as a question type in a form.
1. Click on +Add New
While in the form builder, click on +Add New in the bottom left corner.
2. Click on Question
A drop down menu will appear with some options. Click on Question.
3. Select the Question Type
4. Click on Smart Field
A drop down menu will appear with a few options. Click on Smart Fields.
5. Select desired Custom Field
Select the desired Custom Field by clicking here.
Another drop down menu will appear of available Smart Fields, with existing Custom Fields beneath it.
Just select the Custom Field you're wanting to use, and you are good to go!