• Add a FeatureReader.
• Choose "Esri ArcGIS Server Feature Service" (yes I know, you wrote mapservice, but it works)
• Enter mapservice url (like https://myarcgisserver.com/arcgis/rest/services/a/b/MapServer)
• Hit parameters, choose provide token, select attribute with token.
• Select the layer(s) at FeatureTypes To Read. (You now have to copy paste a valid token to scan the mapservice for layers.)

Tip: I always choose "Single Output Port" over "One per Feature Type" and use an AttributeExposer to Expose attributes. (you can import them instead of entering manually). If you don't, you need to provide a new valid token every single time you make a change in the FeatureReader.