Icons can be added to most text type fields, like Text, Name, Phone, Password etc. and also to the Dropdown field if you want to use that in your form.
To add an icon, click on the Field Icon button in the Appearance tab for the field. Here you can choose any icon from the icon sets or a custom icon from another iconset you may have added. You can also choose a unique color for your icon if you want to. It is also possible to use an image in place of an icon. The Field Icon button opens the Image / Icon Selector.
Icon Placement
The icon can be placed before or after the field, above or even inside. You can find the options to adjust all of this in the Style Selector for your field.