Parsing Value with External Script

I have a question. I need to pass a value to Framed-Pool attribute only to one user, using an external script. My file /etc/freeradius/users user_test Cleartext-Password := "testing" Auth-Type := Accept Framed-Protocol = PPP Framed-Pool = `%{exec:/bin/echo foo}` Fall-Through = Yes It's not working. What it's happening? Using the freeradius debug the Framed-Pool attribute is appearing empty.