src or source_key property to create the asset from an existing file.curl --location --request PUT 'https://demo.myshopify.com//admin/api//themes//assets.json'{
    "asset": {
        "key": "templates/index.liquid",
        "public_url": null,
        "created_at": "2010-07-12T15:31:50-04:00",
        "updated_at": "2023-01-03T12:39:59-05:00",
        "content_type": "application/x-liquid",
        "size": 110,
        "checksum": "cd71db2e14df976c8aa44b44c8dae77b",
        "theme_id": 828155753
    }
}