Here is a procedure to configure RADIUS on Citrix NetScaler version 12.0. For other NetScaler versions see the Citrix Gateway documentation at http://docs.citrix.com and search for the topics you need.
https://docs.citrix.com/en-us/netscaler-gateway/12/authentication-authorization/configure-radius/ng-radius-configuration-tsk.html
Here is a procedure to choose Password Authentication Protocol (PAP) on Citrix NetScaler version 12.0. For other NetScaler versions see the Citrix Gateway documentation at http://docs.citrix.com and search for the topics you need.
https://docs.citrix.com/en-us/netscaler-gateway/12/authentication-authorization/configure-radius/ng-radius-authen-protocols-con.html