Click on the 'Design' tab that you see in the left section of Popupsmart Builder and click on the form fields that pop up on the right.
Make the pop-up window display form field active, and add a new form field as shown below.
To add another form field, click on the '+ Add Now' button and add a new field.
You can see it as marked in the image below. You may not add an extra form element in each theme that is perfectly optimized as a design.