Does QAlert allow you to define symbology and labeling? (I'm not familiar with it.) If not, you'll need to enable labels as part of your service's default symbology:
- Go to the Item in AGOL's web interface
- Open the Visualization tab
- Using the toolbar on the side, open Labels and create one or more label classes
- Save your changes using the button in the upper left
A potential downside here is that this will be your default symbology for every client that uses this service. if you don't want to make these labels universal, first create a hosted feature layer view from your base data, apply the labels to that view, then use that view from QAlert.
Edit - Should mention I'm assuming QAlert uses your default symbology and that it doesn't override it with its own, clients can do that with feature services. If it overrides it, not sure what your options are.