I tried to send the example request in the guide on Custom Route Extension API with my app_id and app_code inserted, shown below. I sent it by entering it in my google chrome browser.
http://cre.cit.api.here.com/2/overlays/upload.json?map_name=OVERLAYBLOCKROAD&overlay_spec=[{"op":"override","shape":[[50.10765,8.68774],[50.10914,8.68771]],"layer":"LINK_ATTRIBUTE_FCN","data":{"VEHICLE_TYPES":"0"}}]&storage=readonly&app_id={YOUR_APP_ID}&app_code={YOUR_APP_CODE}
Indented for readability:
http://cre.cit.api.here.com/2/overlays/upload.json?
map_name=OVERLAYBLOCKROAD&
overlay_spec=[{"op":"override",
"shape":[[50.10765,8.68774],
[50.10914,8.68771]],
"layer":"LINK_ATTRIBUTE_FCN",
"data":{"VEHICLE_TYPES":"0"}}]&
storage=readonly&
app_id={YOUR_APP_ID}&
app_code={YOUR_APP_CODE}
But I get this fault code:
{"faultCode":"s8c73866c-b4a0-4078-90b7-b4d7529c947d",
"responseCode": "403",
"message":"Feature level access has been denied"}
Is Custom Route Extension API not available for trial users?
On their website under "pricing" you will see that custom route extension is indeed a paid feature.