Does the machine have limit switches? If not, it’s not possible to home it automatically. If it does have limit switches installed, the GRBL docs cover homing setup quite well: Grbl v1.1 Configuration · gnea/grbl Wiki · GitHub
If you don’t have limit switches, you’ll have to do it manually. This topic covers machines without limits / homing toward the bottom: Configuring a 3018 CNC (or other CNC-based laser) for use with LightBurn