The issue is the json flag needs to be in the query submitted. if you go to one of the forms:
At the end of the url if it has this parameter &f=pjson it should accept and receive json>?
(I don't think this is technically an arcgis thing, but how react and http requests work)
So if you submit to the query? an expression like:
FeatureServer/7/query?where=1%3D1&objectIds=&time=&geometry=&geometryType=esriGeometryEnvelope&inSR=&spatialRel=esriSpatialRelIntersects&resultType=none&distance=0.0&units=esriSRUnit_Meter&relationParam=&returnGeodetic=false&outFields=*&returnGeometry=true&returnCentroid=false&returnEnvelope=false&featureEncoding=esriDefault&multipatchOption=xyFootprint&maxAllowableOffset=&geometryPrecision=&outSR=&defaultSR=&datumTransformation=&applyVCSProjection=false&returnIdsOnly=false&returnUniqueIdsOnly=false&returnCountOnly=false&returnExtentOnly=false&returnQueryGeometry=false&returnDistinctValues=false&cacheHint=false&orderByFields=&groupByFieldsForStatistics=&outStatistics=&having=&resultOffset=&resultRecordCount=&returnZ=false&returnM=false&returnExceededLimitFeatures=true&quantizationParameters=&sqlFormat=none&f=pjson
that ends in &f=pjson that should return json
note, this only works if the endpoint, the featureserver REST Url is able to be called publicly, if it requires authentication of some kind (shared not with all, but only with certain groups or users), then you need a valid token for that group or user added to the query string)
Additional things to consider:
When you do a query via a rest URL, the parameters submitted in the 'query string' are encoded. the first query parameter is prefixed by a ? followed by a keyname={value} (where {value} is the actual value braces not needed.
So a common one I do is go to the where section and put 1=1 (so its true)
this would encode as:
where=1%3D1
and so on, I think there has to be a valid query, and you need at least one outfield set for it to work. (if your service doesn't use outfields, then it might be different, but most I've seen have that as a parameter for the query)
The last project I did something like this I built the query in code, into one string and tacked it into the baseurl and feature/url in the manner above to make the call.
