To be honest, I am really surprised that you are able to use Rebex Terminal to communicate with AS400. I think, that your AS400 must be using VTxxx instead of TNxxxx, because VT and TN have very different escape sequence specifications.
I think that your AS400 automatically swapped to VTxxx when it received terminal type related to VT family (`xterm`, `DEC-VT100`, `vt100`, or whatever was used).
In your original question, you asked how to specify blank value for 'Workstation ID'. Since we don't use 'Workstation ID' at all, it should be blank by default. So, you probably want to know the value, which was generated by System i?
In such case, I would just try to print it out like other environment variables e.g. `echo $TERM`
I would use: `echo $WORKSTATION_ID`
or: `echo $WORKSTATIONID`
or print all environment variables using the `printenv` command and find the correct name manually.