HOW TO use user parameters to set another user parameter?

[Pavel Gouchtchine]

Hi All.
I am using User Parameter pre-processor to set the following:
host = qatesthost.com <http://qatesthost.com>
protocol = http

Now, I want to combine these variables into variable named qa_url using User
Parameters pre-processor like this:

qa_url = ${protocol}://${host}


JMeter always sets qa_url as "${protocol}://${host}" not as
http://qatesthost.com


Does anybody know how to make qa_url variable use pre set values from other
variables?

Thank you.
Pavel