In the code its calculating the voltage and relative humidity and displaying in the serial monitor
Do you recognize the code that is doing this?
Perhaps this bit:
void printSensorData(HIH4030 hygrometer) {
Serial.print("Sensor Voltage = ");
Serial.print(hygrometer.vout());
Serial.print("V Relative Humidity = ");
Serial.print(hygrometer.getSensorRH());
Serial.println("%");
}
Store the values somewhere, and print the stored values. Then, you can use those same values when sending the data to the server.
How do I send to Xively the relative humidty value?
"How do I use a library that no one has heard of to do what I want?"
Perhaps you start with posting a link to the library. Just a thought.