You can use the MF370CTL.CFG configuration file to set the varable=value pairs that the assembler uses. You can use a MF370CTL.CFG file to maintain a project's variables in one place.
By default, this configuration file is located in the %ProgramFiles(x86)%\Micro Focus\Enterprise Developer\bin directory.
It can be helpful to copy the assembler configuration and control file, MF370CTL.CFG, into your working directory and edit the environment variables in the file to customize where input and output files should be located in your project.