HOW TO
There is a distinction to be made between the custom parameters that can be added at the point of bringing a service into an ArcGIS Online web map viewer, and the parameters that can be added after the service is already active within the web map.
As suggested by the instructions in the Add Layer dialog box shown in the image below: "Add parameters that need to be appended to requests to access this service.", the custom parameters that may be entered from the Add Layer dialog are generally required to access the service itself.
The custom parameters that can be added after having brought a service into the map can be used to query the service layers.
As these are both referred to as custom parameters, there is room for confusion when users are looking to determine how to best use them.
It is recommended to consider the former of the two (when adding the service to a web map) as access-based custom parameters, whereas the latter might be considered query-based custom parameters.
See the following link for various request parameters that may be used as a service-querying parameter: ArcGIS REST APIs: Query (Map Service/Layer)
Article ID: 000032317
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