neutron-lib/api-ref/source/v2/samples/routers/router-update-external-gateways-request.json

{
   "router" : {
      "external_gateways" : [
         {
            "enable_snat" : true,
            "network_id" : "8edec774-ebb9-11eb-9b09-371108ef5905"
         }
      ]
   }
}