I don't have that property in my deployment, so I'll do one using "Number of Processor Cores - Windows" from the "Hardware Information - Windows" Analysis.
Doing API queries from the command line is tricky in that you need to URLencode the query, because quotes, ampersands, percents, spaces, etc. are interpreted by the Windows command shell or curl.exe differently.
The easiest way to figure out the right query is using Web Reports' hidden "QNA" page, at https://your-web-reports-server/webreports?page=QNA
That page behaves like the Fixlet Debugger, but for doing Session Relevance.
Once you have the right query, you can URL-encode it into the command line for Curl, submit it in the URL field on a browser, etc. but I think the easiest way to handle it is to put the query into a file, and then post the file with curl.exe (at least for testing).
In my case I create a file "query.txt" with the following content:
relevance=values of results (bes properties whose (name of it = "Number of Processor Cores - Windows"), bes computers whose (name of it as lowercase = "my_host_name"))
Then, I'll use the curl.exe command line
curl -k -u [myoperatoraccount] -X POST "https://[my_root_server]:52311/api/query" -d @query.txt
And get back a result
<?xml version="1.0" encoding="UTF-8"?>
<BESAPI xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="BESAPI.xsd">
<Query Resource="values of results (bes properties whose (name of it = "Number of Processor Cores - Windows"), bes computers whose (name of it as lowercase = "my_host_name")) ">