Proxy FTP on SEPM

Hello to all the community,
I have a problem with FTP Proxy settings on a Symantec SEPM...I have to use  a Proxy FTP (mandatory and without HTTP connection) to connect LiveUpdate server for  download signatures and I have to use the credential of authentication (mandatory).
The problem is  that in parameters (admin->server->properties->proxy settings) don't I have the possibility to insert userID and pwd (see the enclosure picture).

Do you have any suggestions?

Thank so much in advance, regards

Giordano