With phoenix_swagger you probably want to refer to the Swagger 2.0 Documentation. You have to define a request body as described here: Describing Request Body | Swagger Docs.

Did you check out the example they provide in the repository?

https://github.com/xerions/phoenix_swagger/blob/250d7c79459227095c0eb5c12237afb1b21b6477/examples/simple/lib/simple_web/controllers/user_controller.ex#L93

This could look like the following example

  swagger_path :create do
    post("/api/campaigns")
    description("Create a campaign")

    parameter(:campaign, :body, Schema.ref(:campaign), "Campaign Attributes")

    response(200, "Success")
  end

Additionally you have to define the :campaign schema. Here you can make use of the swagger_definitions/0 function, which you can also add into the same controller:

  def swagger_definitions do
    %{
      campaign:
        swagger_schema do
          title("Campaign")
          description("A campaign.")

          properties do
            data(
              Schema.new do
                properties do
                  name(:string, "Campaign name", required: true)

                  start_time(:string, "Start time of the campaign.",
                    required: true,
                    format: "date-time"
                  )

                  id(:string, "Auto generated ID of type binary_id", required: false)
                end
              end
            )
          end

          example(%{
            campaign: %{
              name: "Marketing 101",
              start_time: "2030-07-01T21:00:00Z"
            }
          })
        end
    }
  end