labelExpression

Simple expression describing how to build the text label for a feature, often using attributes of the feature. The default is an empty expression and no labels will be produced.

The Simple expression format is described at [https://developers.arcgis.com/documentation/common-data-types/labeling-objects.htm] e.g., "\"Addr: \" CONCAT Name CONCAT \" Street\"" (the outer quotes are not part of the expression, but quotes are needed around the literal text).