If you need a SAS macro variable, an autoexec SAS file is appropriate. If it is to be used in the whole environment (DI, EG, batch), add your variable definition in the ../Levn/SASApp/autoexec_usermods.sas file.
If you prefer a system variable, you can set it in either a SAS config file, or a SAS shell/cmd file. They are also available under your configurations directories.
Data never sleeps