There is a need to use a port that is not listed as a common port for the different protocol options found in Common ports for protocols used in VAMI based backups.
FTPS uses port 21 by default, for example.
vCenter 8.x
The protocol, server address, and port number can be specified by logging into the VAMI with the root OS user at https://vcenterfqdn:5480>Backup>CONFIGURE, which pulls up the Create Backup Schedule wizard. The "Backup location" field can be filled out like protocol://yourserver:<port>/folder/location.