Since WAB is linked with AGOL/Portal for ArcGIS, you need to configure pop-ups in web map.
To configure the attributes within the field or fields you want to display, click the Configure Attributes link. Select which fields you want to display. You can also select a field to change its alias, order it, and format it. For number fields (such as population), select the number of decimal places to display and whether to use a 1,000 separator.