labelExpression property

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).