This page will explain how to add labels to a feature layer in a web map in ArcGIS Online (AGOL)
Navigate to map in Map Viewer
Select More Options from the desired layer
Select Create Labels from More Options
See the Label Features window
Choose desired label from the Text drop-down
Note: The drop-down options consist of attributes. A new expression can be created and applied. The expression must be written using Arcade expression language. You can read more about Arcade expressions at this link.
In this example, the CreationDate attribute was chosen in the Text drop-down
Press OK once the labels are set
Click Save
See the end result of the labelled polygons