About authorizing only users with specific attributes (post RADIUS proxy)

I am building RADIUS Proxy with FreeRADIUS 3.0.4. @mydomain.edu -> backendradius.mydomain.com All Other domains -> radius.otherdomain.com RADIUS server of mydomain.com which is its own domain is already built separately, and the server under construction wants to do RADIUS Proxy with the above two rules. For mydomain.com only, I would like to authorize users who have a specific character string in the RADIUS standard attribute Filter-Id. In this case, where should we determine the value of Filter-Id in the RADIUS response from backendradius.mydomain.com? Would you please let me know if you have any reference information, such as a conditional expression with unlang. Thank you.