iconFormat.json

{
  "$schema": "http://columnformatting.sharepointpnp.com/columnFormattingSchema.json",
  "elmType": "div",
  "children": [
    {
      "elmType": "span",
      "attributes": {
        "iconName": "Balloons"
      },
      "style": {
        "padding-right": "6px",
        "color": "#0078d7"
      }
    },
    {
      "elmType": "span",
      "txtContent": "@currentField"
    }
  ]
}