How can I get Recv Percent Utilization from HTTP API call?

Receive percent utilization is a property of interfaces, not nodes.The relevant property is Orion.NPM.Interfaces.InPercentUtil. You can get the percent utilization numbers for all of the interfaces on a node with a query like this:

SELECT InPercentUtil AS RecvPctUtil FROM Orion.NPM.Interfaces WHERE NodeID=103

If you just want to find out the utilization of the busiest interface on the node, you could use this instead:

SELECT MAX(InPercentUtil) AS RecvPctUtil FROM Orion.NPM.Interfaces WHERE NodeID=103

To run this query through the REST interface, POST to https://myhost.com:17778/SolarWinds/InformationService/v3/Json/Query with a body like this:

{"query":"SELECT MAX(InPercentUtil) AS RecvPctUtil FROM Orion.NPM.Interfaces WHERE NodeID=103"}

Or, using query parameters, a body like this:

{
  "query": "SELECT MAX(InPercentUtil) AS RecvPctUtil FROM Orion.NPM.Interfaces WHERE NodeID=@node",
  "parameters": {"node": 103}
}

Okay, this makes sense. I'm trying to run a command:

curl -k -u user:password -X POST -H "Content-Type:application/json" -d "{\"query\":\"SELECT InPercentUtil AS RecvPctUtil FROM Orion.NPM.Interfaces WHERE NodeID=126\"}" https://mysite.com:17778/SolarWinds/InformationService/v3/Json/Query

which I expected was correct, however, I get no output from the command, not even an error. Any suggestions?

When I get no output from curl, my next step is always to add "-v" to the command line so I can see everything that happens, including the TLS details all of the request and request and response headers. Try that and see if it helps.