Selector form meta

Little confused with the selectors when getting from the Header meta, its this possible i’ve tried but didnt work my code is:

{
    "rules" :
        [
           {
                "class": "ImageRule",
                "selector" : "//head/meta[@property='og:image']",
                "properties" : {
                    "image.url" : {
                        "type" : "string",
                        "selector" : "//meta[@property='og:image']",
                        "attribute": "content"
                    }
}
}
]
}

i’m doing it wrong or dont getting the selector stuff
Thanks

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