Hi
This depend on which Radius server you're using. If its ACS then.... ACS will send the cisco-av-pair automagically if
1) the aaa client is defined as Cisco IOS
2) you use the ip assignment setting at group/user level
Unfortunately it wont send a netmask as well. So in the group page just scroll down until you see the "RADIUS Cisco IOS" section then locate the cisco-av-pair and just enter
"ip:netmask=foobar"
right in the text entry box.
Then submit + restart and it should all work.
Darran