How do I add Custom Perf Counters that start with 'R' using the API?

I'm trying to add a Custom Perf Counter using the API using the following URL.

https://prtg/api/setobjectproperty.htm?id=3487&name=counters&value=
Category\Reads+Per+Second

Whenever I use this URL, PRTG treats the \R (at the beginning of "Reads") as a carriage return. When I view the settings for the sensor, the List of Counters setting looks like this:

Category eads Per Second

I've tried escaping the slash (
R) and url encoding the slash (%5CR), but nothing helps. It's always treated as a carriage return. Is there something else I can do to get around this?