how can I add a custom key for a request param in swagger schema definition

adding like this throws error:

should not have additional properties additionalProperty: cusotm_property

"getRequest": {
"type": "object",
"properties": {
"key1": {
"type": "string"
},
"key2": {
"type": "string"
}
},
"cusotm_property": {
"key2": {
"p1": 1,
"p2": 2,
"p3": 2
}

}
},