According the FortiSwitch you have to extend the Fortinet Radius Disctionary by the value of "Fortinet-Host-Port-AVPair". Than you have to create a Enforcement Profile with the value of one of the messages describted in the list - https://docs.fortinet.com/document/fortiswitch/7.2.8/administration-guide/110309/radius-change-of-authorization-coa
Also include the IEEE Radius Attributes in the list in the Enforcement Profile.