Adding support when generating g-code from Cura Engine (command line)?
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I found this in the fdrmprinter.def.json which you could might make a copy of and use instead for the -j option:

"support":
    {
        "label": "Support",
        "type": "category",
        "icon": "category_support",
        "description": "Support",
        "children":
        {
            "support_enable":
            {
                "label": "Enable Support",
                "description": "Enable support structures. These structures support$
                "type": "bool",
                "default_value": false,
                "settable_per_mesh": true,
                "settable_per_extruder": false
            }
        }
    }

Change the "default_value" under "support_enable" to true and that could work.