The verb to accomplish this is 'SET-ENV TCL "string"'
If 'string' is omitted then the TCL prompt will be reset to its default of a ':' (colon)
e.g. SET-ENV TCL "SystemA" will set the TCL prompt to "SystemA:"
Please note this is only valid for your logon session, therefore in order to make this a permanent change you will need to modify or create LOGON PROCS to accomplish this.