How I can add sub node of json in requestbody

Hi All, 

openapi: 3.0.0

I am completly new to swagger, I am creating a swagger resource to share, I need to pass requestbody  as shown below for  a post method.

{
    "field1": {
        "field1_sub1": "3233"
    }
}

How I am doing but not able to add sub node

content:
  application/json:
   schema:
     type: object
     properties:
         field1:
         type: string
              

My question how I can add the sub node in requestBody, so my request is in node->sub node format?

Kind Regards