Render fields for additional properties when using ` application/x-www-form-urlencoded`

Content & configuration

Swagger/OpenAPI definition:

openapi: 3.0.3
info:
 title: additionalProperties Demo
 version: '1.0'
servers:
 - url: /
paths:
 /example:
   put:
     summary: Put endpoint
     requestBody:
       $ref: '#/components/requestBodies/PutRequestBody'
     responses:
       default:
         description: success
components:
 schemas:
   PutRequest:
     type: object
     properties:
       required:
         type: string
       not_required:
         type: string
     required:
       - required
     additionalProperties:
       type: string
 requestBodies:
   PutRequestBody:
     required: true
     content:
       application/x-www-form-urlencoded:
         schema:
           $ref: "#/components/schemas/PutRequest"

Swagger-UI configuration options:

 SwaggerUI({
   '/spec.json',
   dom_id: '#spec'
 })

SwaggerUI version: 3.52.0

Is your feature request related to a problem?

When the request body schema contains additionalProperties and the content type is application/x-www-form-urlencoded, the custom (additional) properties are not rendered on the form (see screenshot), thus making it impossible for the user to enter custom properties to the API call.

The fact that the schema is not referenced in the request body section at all also makes it difficult for the user to even realize that they may add extra properties (and if they do - they would need to add them manually in their curl)

Describe the solution you'd like

If additionalProperties is configured in the schema of the request parameters, and the content type is application/x-www-form-urlencoded, then I would expect some interface to actually add custom fields: maybe a button to add an element, and have both the field key and value empty. See screenshot:

Describe alternatives you've considered

The workaround for this would be to just specify this in text in the request body description.

Additional context

Reproduce the issue in Swagger UI with the provided OpenAPI 3.0.3 definition and an application/x-www-form-urlencoded request body containing additionalProperties. Trace the form rendering and request serialization entry points; done means users can add custom key/value fields and those fields appear in the generated API call.