I need to insert a polygon from GeoJSON to my PostGIS table. This is how the SQL query looks like.

INSERT INTO tablename (name, polygon)
VALUES (
    'Name',
    ST_GeomFromGeoJSON(
        '{
            "type": "Polygon",
            "coordinates": [
                [7.734375,51.835777520452],
                [3.8671875,48.341646172375],
                [7.20703125,43.580390855608],
                [18.6328125,43.834526782237],
                [17.9296875,50.289339253292],
                [13.7109375,54.059387886624],
                [7.734375,51.835777520452]
            ]
        }'
    )
)

Unfortunately, I get an error message.

ERROR:  Geometry SRID (0) does not match column SRID (3857)

The GeoJSON is already in the right reference system. But this isn't specified. How do I specify the SRID in the GeoJSON? What does the GeoJSON need to look like?

Update: When I wrap the geometry created by ST_GeomFromGeoJSON with ST_SetSRID(..., 3857) it throws another error. In my view it doesn't seem that the geometry has a Z dimension.

ERROR:  Geometry has Z dimension but column does not